How to avoid 'old tank syndrome'

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How to avoid 'old tank syndrome'

Older aquariums can have issues of their own, leading to dead fish and terrible water. The tragedy is that it’s so simple to avoid this kind of problem, says Nathan Hill.


If you’ve kept fish for anything more than a couple of days, then you’ll be either directly or indirectly intimate with the problem of new tank syndrome, often abbreviated to NTS. 

I won’t dwell. We should all know the basics of new tank syndrome, though discussion about the exact mechanisms is heated. Fish make waste, the filter struggles to cope with that waste, and contingency plans need to be in place. You might prefer ex-situ tank maturation or in-situ. These are arguments for another time.

Unfortunately for many fish, too many aquarists assume that new tank syndrome is the only water worry they’ll ever face. They imagine that this initial establishment period is the only time they’ll struggle with instability.  

Because of this, they may be caught out by an equally dangerous, yet easily avoidable problem later down the line: old tank syndrome, or OTS.

In old tank syndrome, the definition of 'old' is open to some discussion. Tanks that have been running successfully for months or years may slowly turn bad, or they might suddenly crash altogether, with multiple casualties and a befuddled owner. 

The problem all too frequently lingers unseen, and then manifests itself in a way that leads to conflict between fishkeeper and retailer.

Imagine the scene, if you will. A fishkeeper has a tank that they’ve kept successfully for two years. In that time, some fish have grown, while others — conceivably through old age — have died. The plants are large, maybe a little leggy, but still alive, and overall there is no reason to suspect any underlying malaise. 

This fishkeeper opts to buy some more fish from a local store, deciding to pick up some delicate tetra types, given how 'mature' their tank is. A day or two after adding the fish, problems are noted. They look bedraggled, and show the first signs of whitespot. Not long after, the tank is knee deep in a full scale Ichthyopthirius pandemic, with old and new fish dying everywhere.

The immediate impression of the fishkeeper is that they have been sold diseased fish, and so they return to the store to confront the retailer and demand replacements — only to discover that every one of the original tetras still in store is the absolute picture of health. 

As is their right, the retailer requests a sample of the fishkeeper’s aquarium water before a resolution can be made, and that sample, when it arrives, turns out to be diabolical beyond compare. So bad, in fact, that even without the new fish, the tank would have been weeks away from a wipeout anyway.

And that is how most people meet old tank syndrome. 

 Cardinal tetra with whitespot.

Cardinal tetra with whitespot.

More to water than filters

If I was going for an easy sale in a store, and I had to describe a filter’s function as simply as possible, then I might be inclined to say that 'it takes all the toxic waste and makes it safe for the fish to live with'. I suspect that for a lot of casual fishkeepers, that’s the impression they have in mind. Fish make waste, filter resolves waste, problem solved. 

If only it was that that simple. 

Filters only convert waste, and certainly not into something safe. Ammonia from fish is converted through nitrite and into nitrate. That much we all know, right? Water chemistry 101 right here. 

Nitrate isn’t exactly a 'nice' chemical to have around. In humans, it’s implicated as a carcinogenic, while the lethal levels for different fish are slowly being understood. 

As long as the filter is working, and as long as the fish are producing waste, then there will be nitrate produced. 

It would be wrong to say that some fish develop a tolerance for nitrate, because that implies that they can be unharmed by it. Rather, I consider long-term nitrate exposure in fish as analogous to alcoholism in humans. It slowly affects organs, reduces lifespans, and plays havoc with immune systems. To be in optimal health, they need to live lives devoid of it. Like drink in humans, a little bit of it might not be a major problem. A lot of it is. 

The problem is that aside some slight loss of condition, maybe dull colours or lethargic behaviour, there’s little to see that suggests a fish is suffering in high nitrate levels. 

Even worse, if a fish is then taken from water where there is very little nitrate (such as in the case of my hypothetical retailer earlier) and suddenly exposed to high nitrate, then it is likely to shock it. To stay with the drinking analogy, someone unhealthy but used to drinking heavily would cope better with a ten pint boozing session than a person in perfect health who has been a lifelong teetotaller. Think Rab C. Nesbitt versus a nun. It’s a crude way to explain it, but that’s pretty much what happens. 

There’s also phosphate to consider. Phosphate is introduced courtesy of the foods we offer our fish. It is not metabolised to any degree, and ends up excreted as waste. 

Phosphate has questionable health impacts on fish, and is much better understood to inhibit invertebrates. Marine keepers in particular struggle to control it. As phosphate builds, the most obvious symptom is increased algae growth. In waterways, excess phosphate leads to eutrophication (it is the prime cause of eutrophication), which turns rivers and lakes bright green with algae, stripping the water of oxygen and killing the inhabitants. Such pandemics are not well known in aquaria, though that’s not to say they never happen. 

Again, all of this is very easy to avoid. 

The acid effect

The worst culprits for OTS are tanks that become victims of their own success. 

An underlying problem involves the hardness — the mineral content — of the water. Carbonates play a vital role in buffering aquarium pH, as well as providing a source of carbon for some plants and bacteria: specifically those bacteria helping to convert fish waste. 

When carbonates become depleted, two problems follow. Firstly, the osmoregulation of many fish (the way in which they regulate their minerals in the body) is compromised, sometimes to lethal extents. Secondly, without the buffering effect of minerals to keep it in place, pH in the tank can drop or swing wildly, leading to acute acidosis. Either of these can kill a fish outright, and both will cause acute and chronic stress.

Bacterial action inside the filter further alters pH. Biological bacteria, as they work, produce quantities of hydrogen ions. In water, these take the form of hydronium, and the amount of hydronium relative to the amount of hydroxide dictates how acidic the tank is. The more hydronium, the lower the pH, and with a busy filter churning the stuff out, pH levels can soon plummet. Combine that with the simultaneous removal of buffering carbonates as already mentioned, and you have a pH disaster in the waiting. 

To make things even worse, if the pH does drop too far then it will stop the filter from working properly. In acidic conditions, the bacteria that convert ammonia struggle to function, and below 6.0pH they might stop working altogether.  

Prevention trumps cure

Every parameter mentioned here can be tested, inexpensively and easily. Liquid kits, dip strips, monitoring devices — there is no excuse. Folks who say they can visually assess their tanks are deluding themselves, at the peril of their fish. You cannot ‘see’ the nitrate content of a tank any more than I can 'see' the alcohol content of a glass of gin. 

Over the years, there has been something of a misunderstanding. Many aquarists are only keen to test — or in some cases have their retailer test for them — while they are cycling the filter at the start of the tank’s life, and wrongly assume that once that’s out of the way it’s plain sailing. 

Testing for the chemicals associated with old tanks needs to be performed weekly, or fortnightly at least. Nitrate, GH, KH, phosphate and pH are all reliable indicators for the health of your tank. At an absolute minimum, nitrate and pH need to be closely watched. 

Start by testing your water supply, whether you use tapwater or RO mixes purchased from a store. Unless you are deliberately altering the water in the tank with acidic products like Catappa leaves, then a difference of 0.5pH between the aquarium and the water source is a huge cause for concern. Nitrate levels always need to be low, but if the tank water tests at 40ppm greater than the water source, alarm bells should be ringing. 

If you test for phosphate, then levels in the tank should be no higher than 0.5ppm greater than those in the water source. At 5ppm or more, action is essential. 

GH and KH levels in the tank should be no more than a couple of degrees lower than source, though it’s likely that KH will deplete faster out of the two. Unless you’re keeping incredibly softwater fish, a tank reading below 5°KH is a warning that cannot be ignored. 

Testing is one thing, but the actual process of prevention counts for even more. 

The way these issues are avoided is simple: water changes, gravel cleaning and maintenance. There’s no short cut here, and no way to avoid the inevitable. Old tank syndrome is caused by a lack of tank care, plain and simple. 

Water changes will fix most things. Taking old water out and replacing it with fresh will dilute down the levels of nitrates and phosphates. Because the newer water will be richer in minerals, it will also help to boost hardness, increasing the GH and KH, and stabilising pH. 

If the tank is in a bad way, then opt for a course of small changes — one won’t be enough. Changing 25% daily or every other day will slowly bring things back to where they need to be. 

If things really are extreme, to the point of a tank crash, you may want to perform a larger change of 50–75%, though this is a drastic measure. Adding a mineral product such Tropic Marin Remineral Tropic will help instantly boost the GH and KH. An excess of ammonia in the water may be treated with the likes of API Ammo Lock. Nitrates and phosphates can be reduced by using dedicated resin media placed into the filter. 

Essentially, all of these points need to be tackled at once. Addressing nitrate levels while the pH plummets is like washing a car while the engine is on fire.

Hidden waste

Cleaning gravel is paramount to avoiding OTS. All of the 'hidden' waste down amongst the substrate will gobble up carbonates, and churn out more nitrates than a rotting tree. Left too long, aquarists can even face the rare but lethal hazard of hydrogen sulphide production, where oxygen levels in the gravel drop so low that the 'wrong' kinds of bacteria can proliferate. 

A simple gravel syphon will keep on top of this collected slurry. Using a cleaner like the Fluval AquaVac+ will pull up much of it, though it’s just as easy to combine gravel cleaning with a water change and kill two birds with the same stone. 

Maintenance is essential, especially for the filter and any plants. Filters accumulate waste much like the gravel does, while some media which removes the likes of nitrates can become exhausted and release much of what it has trapped back into the tank. Frequent changing of resin media, and regular cleaning of sponges is vital.

Plants can also contribute to OTS if kept poorly. Old leaves may drop or degrade, and then they’ll start to rot — meaning more carbonate depletion and more nitrates!

By combining regular water testing with regular water changes and maintenance, there’s no reason for anyone to suffer from old tank syndrome.

You never know, those basic chores might even save you from having an embarrassing fracas with your retailer one day over dead fish..

Potential causes of OTS

  • Inadequate water changes.
  • Overstocked fish levels.
  • Dirty gravel.
  • Uneaten food.
  • Unwashed filters.
  • Dead fish/shrimps/snails.
  • Decaying plant matter.
  • Low GH or KH water supply.
  • High nitrate water supply.