Reef chemistry as it happens!


What scientific reactions are taking place in our reef tanks? Levi Major unravels some chemical mysteries.

How does calcium carbonate form in my tank?
This is created in the aquarium by two main processes. Firstly, calcium and carbonate ions can be locked by living organisms, such as coral or coralline algae, within their calcium carbonate skeletons. Non-biological (abiotic) precipitation can also tend to occur.

For either to take place there’s a requirement for both calcium and carbonate ions to be in solution generally at or above the solution point.  

This can be taken as the point where the rate at which calcium and carbonate ions in solution land on a solid calcium carbonate surface and come out of solution is equalled by the rate at which they leave to become part of the solution.

If we raised the levels of calcium and/or carbonate within the solution it would be supersaturated — having more ions in solution than can be deemed stable over a long term.

Then precipitation of calcium carbonate can take place because the rate at which ions land on the surface of the solid exceeds the rate at which they can leave. Therefore, the solid surface is built up with further calcium and carbonate ions.

Even though more complex than abiotic precipitation, corals almost exclusively take calcium and carbonate ions from the solution to deposit their skeletons. The chemical composition of calcium carbonate (CaCO3-Ca2+ + CO3-2) indicates these organisms use a 1:1 ratio of calcium and alkalinity.  

However, the consumption of calcium can be seen to vary from species to species, due to the  incorporation of magnesium within calcium carbonate.

If we discover that we have very high calcium or alkalinity in our aquaria, precipitation of calcium carbonate can reduce the level of both in solution.

If we increase the levels of these ions so that they exceed the saturation point then the rate of calcium carbonate precipitation is increased. The result is accelerated coral growth.

Conversely, if the levels of calcium and/or carbonate ions in solution were below saturation point there would be no net gain in precipitation of calcium carbonate.

So should I just maintain high levels of calcium and alkalinity if I want fast coral growth?
Not exactly! The actual rate at which calcium carbonate is deposited can be affected by certain other water parameters, such as pH and magnesium.

Both bicarbonate and carbonate are forms of the same ion. At a lower pH, the bicarbonate form (HCO3-) predominates, whereas at a higher pH more of the carbonate form (CO32-) exists.  

The effect of varying pH can be acute, in that each drop of 0.3 pH units below a pH of 9 causes a two-fold drop in carbonate concentration.  

A full pH unit drop would correspond to a ten-fold decrease in carbonate concentration.

Accordingly, by varying the pH of a solution we can also change the amount of carbonate ion in solution. As mentioned, the concentration of these ions in solution determines the rate at which carbonate ions land on the surface of the solid.  So the higher the pH, the faster the rate at which these ions land. Therefore the solubility of calcium is lower at higher pHs.

Lower solubility implies that as pH increases the amount of calcium and alkalinity that can be kept in solution without precipitation occurring decreases.  

If we were to add kalkwasser (limewater) solution to the aquarium, pH would increase.  This would rapidly permit precipitation of calcium carbonate.

This is not necessarily as a result of increasing levels of calcium or alkalinity, but also due to the fact that as we increase pH much existing bicarbonate within the water converts to carbonate. Then there’s a resultant spike in carbonate concentration.

The opposite is true with a falling pH in that the amount of calcium and alkalinity that can be kept in solution without precipitation occurring increases. This is why adding carbon dioxide to a reactor dissolves the media.

You may think a lower pH is better as you can maintain calcium and alkalinity levels better and abiotic precipitation of calcium carbonate will not occur. However, your corals would have to work harder converting bicarbonate to carbonate to allow calcification.



How does the level of magnesium affect the levels of calcium and carbonate ions in solution? Does it affect the rate of calcium carbonate precipitation?

The role of magnesium is far more complex than pH and alkalinity.  If we examine standard seawater magnesium is the third most abundant ion after chloride and sodium, so its presence complicates our simplistic view of calcium and carbonate ions landing on and leaving the surface of our solid calcium carbonate.

Magnesium ions can be incorporated into the crystal structure of calcium carbonate and replace the role of the calcium ions.

As ever more magnesium is incorporated onto the surface of the calcium carbonate a layer of calcium and magnesium carbonate is formed. This results in the surface of the calcium carbonate no longer resembling one of calcium carbonate, preventing a firm bond of further calcium and carbonate ions. Further precipitation of calcium carbonate is largely reduced.

The extent to which magnesium is incorporated within calcium carbonate surfaces depends on the amount of ions in solution. The greater, the more it is incorporated.

If magnesium levels are lower than normal, it may not adequately get onto the growing calcium carbonate surfaces. This will then allow calcium carbonate to proceed even faster and may lead to increasing abiotic precipitation of the calcium carbonate.

Our inability to maintain adequate calcium and alkalinity levels, despite extensive supplementation or evidence of abiotic precipitation of calcium carbonate on heaters and pumps, means that the levels of magnesium are inadequate within the home aquarium.

Should I use supplements if my parameters seem fine?
Routine water tests will show your parameters and whether you need to supplement your tank beyond a water change. If you know what parameter is out of kilter, you should now know the best way to tinker and tune your set-up.

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Want to try something new in reefkeeping?


Looking for the next challenge? Can we make a success of azooxanthellate coral tanks? Jeremy Gay suggests making a bold move with a future marine project.

We can do soft, LPS and even SPS corals with ease these days, so where is the next challenge for the enterprising reefer? The answer is non-photosynthetic corals.

These include more common species like Sun corals (Tubastrea spp.) but also the brightly coloured and considered near impossible Dendronephthya and Scleronephthya, and the gorgonians such as Acabaria, Acalcygorgia, Melithaea and Subergorgia.

You could be the best SPS coral propagator in the world, yet success with these creatures is far from guaranteed. With non-photosynthetic or azooxanthellate corals it isn’t about light, as you might expect. Instead it’s to do with the right type of water flow, delivered the right way over a 24-hour period, and feeding.

Water flow
Divers and naturalists are aware that gorgonians, or sea fans as they are also known, are usually found at depth, projecting at right angles from rock walls where their branch-like structures collect food particles from passing currents. The flow is known as laminar, moving uni-directionally, and the corals position themselves to expose maximum surface area to catch food. In strongly tidal zones the flow will also reverse over a 12-hour period.

This leaves the reefkeeper facing a most interesting conundrum, as most of us have just spent the last 15 years equipping our aquariums to replicate the brightly lit reef crest where photosynthetic corals grow in chaotic and ever changing flow conditions.

A complete rethink will be necessary to provide the right flow conditions for these plankton-eating corals and, 99 times out of 100, a completely separate tank set-up too.

Flow must be uni-directional,  and not too hard you may think, but that flow must also be capable of keeping the plankton in the water column for as long as possible to give the corals the best chances of capturing them.

Say goodbye to any mechanical filtration, as this will remove the precious food source. Initial trials, placing gorgonians in a tube as part of a closed loop, showed promise, although it will not look anything less than a science experiment if you choose to display them that way in your home.

Capture possibilities
A separate sump-style arrangement with baffles arranged in a similar way to an algal refugium may work, or even the 'manifold' pump and pipework arrangement used by many freshwater loach keepers.

Replace the sponge strainers with more open plastic ones and your plankton may recirculate many times, offering excellent capture possibilities.

Install ball valves in recirc pipework or inline taps into manifolds so that you can control that flow too.

Just to make things more complicated, it’s also about tethering that flow so the polyps can open to their maximum and create an eddy that moves prey towards them. Too much flow and they can’t open or function properly. Much experimentation is needed and, to make things worse, preferred flow velocity varies from species to species!

Zooxanthellate corals derive some energy from the symbiotic zooxanthellae algae that live within their tissues, so secondary feeding of other foods like phytoplankton or zooplankton isn’t as critical as with the gorgonians, and heavy feeding of zooplankton, although benefiting many light-loving corals and other organisms, can add to the biological load and nutrient poor environment we are wanting to create.

Azooxanthellate corals rely totally on food capture for their nutritional needs so — like fish — no food, wrong food or improper diet will end in death.

It was thought that zooplankton was the main requirement for azooxanthellate corals, though smaller phytoplankton are now also believed necessary, along with many other foods like bacteria and nutrients like amino acids.

Luckily they are available, so how much food do we offer and how often?

Provide suitable foods in terms of nutritional content and capture size as and when you can. Some public aquariums even pump in natural seawater 24/7 to try to get adequate results.

A quick Internet search reveals mixed results if considering keeping what many deem impossible. Don’t bother as azooxanthellate coral tanks are too hard, say many industry experts and scientific peers — yet search on and you’ll find photographic evidence of a growing number not only working but appearing to thrive.

That leaves another conundrum: whether to invest time and resources in giving it a go and risking failure, or simply leaving well alone. No one wants dead corals and that is a huge strain on the world’s resources in terms of collection, shipping, transport, packing and energy, but without someone prepared to give it a go we wouldn’t be where we are now with SPS corals!

This item first appeared in the June 2010 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.

Keeping jellyfish in the aquarium


Complicated life cycles ultimately create these simple but beautiful sea creatures. David Wolfenden wonders if jellyfish are about to become a home hobby favourite.

Every now and then, a fair number of hobbyists become interested in keeping jellyfish at home. However, this idea is never sustained and jellies have failed to become as popular as predicted.

They remain, however, a relatively common sight in public aquaria and, with some commercially available systems now appearing, how viable do they now seem for domestic tanks?

Keeping jellyfish is a relatively new branch of aquarium science. Until recently it was thought impossible to keep pelagic (open ocean) jellyfish for any length of time in captivity — standard aquarium conditions proving totally unsuitable.

However, pioneering work in the early 1990s at California’s Monterey Bay Aquarium, along with data gathered in Japanese aquariums, succeeded in maintaining a variety of species, starting with the Moon jelly (Aurelia aurita) which is still the most common jellyfish in captivity.

Success with keeping jellies was due to the development of the kreisel tank (from the German for ‘merry-go-round’), which was originally designed to keep gelatinous planktonic animals alive on research ships and in laboratories. The aim is to produce a gentle, flowing water motion in which the delicate jellies and their food can be suspended.

Many aquarium jelly tanks are more correctly referred to as pseudokreisels as they 'borrow' elements from the 'true' kreisel design but modified for better viewing.

Simple, but complicated
Aurelia aurita is a species of scyphozoan ('true') jellyfish with a more or less worldwide distribution. There is debate about the taxonomy of the genus Aurelia, but we consider it a pretty cosmopolitan group of closely-related populations.

The familiar 'jellyfish' (the 'medusa' stage) is only a part of the life cycle of Aurelia, which is seemingly rather complex for such primitive, simple creatures. Adult medusae reproduce sexually to produce a planula larva which anchors itself to a substrate before becoming an anemone-like scyphistoma —the polyp stage – similar to those of corals, to which jellyfish are related.

This eventually becomes a strobila — each one producing around 15  flattened discs and each of these being an ephyra, a juvenile jellyfish with a star-like appearance and eight bifid ('split') arms released in a process called strobilation. The ephyrae become part of the plankton where they grow into medusae — the reproductive and dispersal stage — and the life cycle turns full circle. Life spans of over four years for individual Aurelia medusae have been reported, although two is a more usual.

Polyps can be maintained simply in a small, bare aquarium with open-ended air line fixed to the base to circulate food — Artemia nauplii, preferably decapsulated to prevent ingestion of the indigestible cysts — and provide gas exchange. Filtration is unnecessary, with water quality maintained through regular syphoning of detritus and subsequent water changes.

'Budding'
Ensure that excessive feeding doesn’t allow hydroids to take over the tank. If maintained at a constant temperature and fed sufficiently, the polyps will reproduce asexually by 'budding' so a small number can give rise to a carpet. Add shells and other objects to which they can attach.

Strobilation in Aurelia occurs after prolonged changes in temperature and some success has been achieved by adding iodine to the culture water, although different ‘strains’ of Aurelia may have different triggers for strobilation.

The resulting ephyrae should be removed from the polyp tank to prevent them being eaten by the polyps and transferred to a similar set-up, the open-ended air line helping to move the juveniles and keep the Artemia in suspension.

This can work well as a 'grow out tank' until the ephyrae have become recognisable as small medusae up to about 2cm/0.8” across — by which time they will need to be transferred to a kreisel tank as air bubbles can start to damage them.

Home comforts
The basic design for a jellyfish tank can be quite simple — although commercially-available systems are available, DIY kreisels can be made with a little ingenuity.

The illustration above shows a simple design for a basic pseudokreisel tank. This successfully housed Aurelia medusae. Filtration was not utilised, water quality being maintained with regular syphoning and water changes, although external biological filtration could be utilised via the rear ('non display') portion of the tank. 

A jelly tank must be radically differently from a normal aquarium. A well-designed kreisel (or pseudokreisel) will create laminar, sheet-like water circulation patterns to keep the jellies in suspension. Flow rates must be gentle to prevent damaging them, but strong enough to keep the animals moving and prevent dead spots where they might collect.

Aeration is not a feature of such tanks, as any air bubbles will become stuck under the medusa’s umbrella, leading to holes in the body. If these aren’t removed rapidly the jellies will die and many a tank has suffered a wipe-out from accidental introduction of air bubbles.

Gas exchange will take place at the water’s surface, even in gentle flow, but ensure that a 'skin' of oil isn’t allowed to accumulate there.

Biological filtration is beneficial provided it can’t introduce air bubbles into the tank and a coarse medium, such as bio-balls, prevents the jellies’ food becoming rapidly depleted — media-like foam tending to filter the feed out too efficiently.

Some basic tank designs don’t employ filtration at all and these can be reasonably successful, although very frequent water changes are obviously necessary — which aren’t a problem for most public aquariums but less practical at home. Protein skimmers are usually considered unnecessary for jellyfish kreisels, as they may clean the water a little too aggressively, not to mention increasing the risk of potentially lethal microbubbles being added to the tank water.

Aurelia are temperate and chilling the water may be another requirement: the optimum range for most strains being 10-15°C/50-59°F, although slightly higher temperatures are often tolerated.

Large medusae will benefit from varied feeding — in addition to decapsulated, preferably enriched Artemia, there’s room for a certain amount of experimentation. The Monterey Bay Aquarium include Pacific krill in the diet of their Aurelia, and Japan’s Osaka Ring of Fire Aquarium report that minced clam as a supplementary feed prevents deformation and promotes healthy growth. Aim for variety in the medusa diet.

Aurelia doesn’t have special lighting requirements, but actinic blue lighting is aesthetically effective, as well as reducing algal growth, so considerably cutting down kreisel maintenance.

If adult Aurelia can be kept for long enough in a kreisel, sexual reproduction may well happen, with polyps appearing in the tank as a result. These can be scraped off the glass with a razor blade and transferred to a culture vessel, although another way is to place half scallop shells on the kreisel’s base. They are flat and smooth enough for the medusae to glide over them, but are an ideal substrate for settlement by the planula larvae. Once a few polyps are on the shell, they can be easily removed and cultured separately.

Catch your own!
Where can you get jellies? You can’t pop down to your local shop to buy Aurelia, which leaves one option – catch your own!

A little bit of 'pioneer mentality' is required if you’re serious about jellies! If able to obtain wild Moon jellies, they are best transported in large, thick plastic bags with all the air squeezed out — and never take a jelly out of the water!

A few polyps are all that’s needed for a starter culture. Using wild-caught animals may not initially be ideal, but our knowledge of their reproductive habits and culture requirements is now sufficient to allow a genuinely sustainable captive population of jellies to quickly become established from a few wild-caught individuals. After all, this is what public aquariums  have been doing for years, with by far the vast majority of specimens on display being captive-raised.

Other species
Other, more ‘exotic’ species of pelagic jelly are successfully maintained in public aquaria and all present unique challenges. One such beautiful species is the Pacific Mastigias papua, which houses symbiotic zooxanthellae, as does Phyllorhiza punctata, so these have additional lighting requirements.

Chrysaora fuscescens, the sea nettle, is also kept by some establishments, but these are large and generally require supplementary feeding on other jellyfish! Whether any of these could be viable for home culture is debatable…

Easier alternative
Easier to maintain in captivity are the 'upside-down' jellies of the genus Cassiopeia, which are  occasionally seen in the trade. Unlike Aurelia or other pelagic jellyfish, this species spend most  time on the shallow sea floor, upside down to expose their symbiotic zooxanthellae to the sunlight.

Adults of C. xamachana can grow to a diameter of 30cm/12”, with a lifespan of around one year, with the more commonly-available C. andromeda reaching 20cm/8”.

Ethusa spp., a group of crabs, are regularly seen on Indo-pacific reefs carrying Cassiopeia jellies on their backs for camouflage and protection. While not as mesmerising as their pelagic counterparts, these can look stunning, often exhibiting delicate blue and purple coloration.

Care for Cassiopeia is more straightforward than for say Aurelia, or any other truly pelagic jelly, although specific requirements still need to be catered for. They can be kept in shallow aquaria —depth being less important than area — with a sandy to coarse substrate, and they do best in a species tank. Many specimens are doomed to die quickly as ‘novelty’ additions to a standard reef aquarium.

Filtration can be achieved via the reverse-flow method, although many public aquaria do not utilise biological filtration to achieve rapid growth rates. Cassiopeia are hardy and tolerant of a range of water quality parameters and detectable levels of ammonia don’t seem an issue. In fact, ammonia supplies the jellies’ algal symbionts with nutrients, resulting in greater growth rates than when biological filtration is employed!

Some aquarists also report that reproduction is triggered by increases in ammonia. It’s vital that Cassiopeia receive intense lighting of suitable ‘reef’ quality, but won’t tolerate currents normally associated with reef aquaria — so strong or even moderate water movement is a no-no!

Feeding on Artemia nauplii —again, preferably decapsulated — and other foods is necessary.

Breeding Cassiopeia in captivity is possible and many public aquariums culture upside-down jellies as a food source for other species. Cassiopeia polyps continuously strobilate at temperatures above 25°C/77°F and polyps kept in a culture tank at a lower temperature will asexually bud off tiny replicates of themselves, so it’s possible to keep a healthy supply of polyps.

Ephyrae and small medusae can be maintained in simple 'grow-out' tanks.

This item first appeared in the November 2008 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.

Pulsing Xenia, Xenia elongata


Pulsing Xenia makes a great addition to the reef tank, and it's a marvel to watch says Jeremy Gay.

Common name: Pulsing xenia.
Scientific name: Xenia elongata.
Origin: Indo Pacific.
Requirements: Moderate to bright lighting from T5, LED or metal halide. It should have moderate flow with ten times tank volume turnover per hour.
Water parameters: Very hardy with regard to parameters and tolerates even nitrate-laden water. (See notes below.)
Notes: Pulsing Xenia is the perfect soft coral for beginners as it tolerates and even thrives in less than perfect conditions.

Some reefkeepers say that this coral actually uses nitrates as a nutrient — and it has even been used in Xenia-only refugiums as it can filter water-removing nitrate and capture tiny food particles as it goes along.

The pulsing action of the large polyps on this coral are the main attraction!

You have to see a live one in action just to appreciate the marvel of this coral opening and closing all its polyps in quick succession, and it always grabs the attention of non-fishkeeping onlookers too.

It pulses to draw oxygen over itself and the less water flow it receives the more it pulses.

However, this coral is considered a pest by some marine keepers, as it can rapidly spread all over the aquarium. However, shops will always take any frags as imported specimens don’t appear to do as well — due to lack of oxygen available in transport.

Availability: Widely available from marine stores and hobbyists.

Price: £30-50, though frags will be £10 or even free from hobbyists.

Montipora capricornis


Also known as Monti for short, Montipora capricornis is a widely available coral, but try to buy frags if you can, says Jeremy Gay.

Common name: Montipora, Monti
Scientific name: Montipora capricornis
Origin: Pacific Ocean
Requirements: Bright lighting from metal halide, multiple T5 or high power LEDs. Strong flow averaging 20 times tank volume turned over per hour.
Water parameters: 24-26°C/75-79°F, sg 1.026, 35 ppt. pH 8-8.4, alkalinity 7-9 KH, calcium 400 ppm, magnesium 1350 ppm. Phosphate and nitrate zero.
Notes: One of the easiest SPS corals to recognise, the orange Montipora capricornis is also one of the easiest to keep. Its plate-like shape and swirls and its bright colour make this a popular addition to reef tanks. It is also a good stepping stone coral if on the way to trying out a full-blown SPS system with more demanding species like Acropora.

In the right conditions it has high calcium demands, so lots of water changes with a good salt, supplements or automated dosing may be necessary. If left unchecked it will form a large plate, 30cm/12” plus in diameter and then start to grow vertically too, taking on its characteristic and desirable swirls.

Its size and shape will mean it will shade corals beneath so place low down in the aquascape, lighting permitting. However, the overhang it creates does look characteristic of a natural reef.

This species is easy to propagate in reef tanks – sometimes by accident as its brittle skeleton makes it vulnerable to breakage. Broken-off bits and frags will continue to grow unaffected, making it also suitable as a fragger’s first SPS coral.  

Availability: Widely available from marine shops and mail order coral websites. Fragged and captive cultured specimens are better in terms of acclimatisation and in protecting the reefs from overcollecting.
Price: £30-50, depending on size. Frags may be as cheap as £10.

This item was first published in the December 2009 issue of Practical Fishkeeping magazine. It may not be reproduced without written permission.