Frequently asked questions on Ultra Low Nutrient Systems


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Want to know more about ULNS? Eric Michael Sanchez has some answers that may help.

What constitutes “ultra low nutrient” and what makes an Ultra Low Nutrient System?

It’s a reef aquarium that maintains very low levels of nutrients. The word 'nutrient' itself is a broad term, but the levels we focus on are nitrate (NO3) and phosphate (PO4).

In an ultra low nutrient environment these should be undetectable on hobbyist grade test kits. For high sensitivity testing, nitrate levels between 0-1ppm and phosphate levels of less than 0.03ppm are generally considered low, or ultra low, nutrient.

Even at these seemingly low levels, when compared to the ocean’s stony reefs we attempt to mimic they are extremely elevated.

An ultra low nutrient environment can be created in many ways. However, the term ultra low nutrient system (ULNS) is most often associated with probiotic or bacteria-driven systems due to their efficiency in reducing nitrate and phosphate levels.

What’s the point and what are the benefits?

Low nutrients are paramount for the avid SPS or Acropora aficionado. Reef aquariums exhibit elevated nutrient levels compared to the oligotrophic (nutrient poor) waters where the SPS corals naturally occur.

Under elevated levels of nutrients, such as phosphate, stony corals exhibit suppressed calcification — reduced growth. This issue can be eliminated in aquariums thanks to modern filtration technology, but colour is another benefit as a result of low nutrients.

SPS corals gain energy from zooxanthellae, their symbiotic algae that is generally gold/brown and can be seen through the coral’s flesh. As nitrogen permeates the coral tissue, the zooxanthellae underneath can quickly utilise it to grow. Elevated nutrients fertilise the zooxanthellae and cause dense levels within the coral tissue, resulting in drab brown corals.

When these nutrient levels are decreased so are the zooxanthellae densities. This allows more coral pigmentation to be shown and less of the colour brown.

With such low levels of nitrogen and phosphate, algal growth can also be significantly reduced.

There is also evidence that bacteria in the water column can serve as a food source for corals. This has prompted some aquarists to incorporate ULNS techniques to help manage nutrient levels while providing that food.

How can my aquarium become ultra low nutrient?

How nutrient levels are reduced and maintained is up to the aquarist, but bacteria driven, ultra- low nutrient systems based on organic carbon dosing have quickly become popular.

Organic carbon is added to the aquarium in stringent low doses to cultivate bacteria in the water and, in some cases, within reactors. This is not without risk but, if done properly, the end result is a powerful nutrient reduction.

Many proprietary systems are based around the idea of organic carbon such as ZEOvit, UltraLith and Prodibio, but even sugar and vodka additions follow the same underlying principles.

How do bacteria lower nutrients?

As the bacteria reproduce from the addition of carbon, other inputs are then required for synthesis, namely nitrogen and phosphate. The bacteria can then assimilate nitrate and phosphate in the aquarium, incorporating it into the growing biomass which is then removed via protein skimming.

The skimmers are vital for exporting the bacteria and utilised nutrients. High levels of nitrate and phosphate have become undetectable after weeks or, in some cases, even days.

This is the basic premise of commercial bacteria-based filtration methods, as well as vodka and mixed carbon source dosing such as VSV (vodka-sugar-vinegar).

Will water chemistry be affected?

ULNS can magnify issues from parameter shifts. Under traditional filtration methods changes in parameters can cause stunted growth or browning. Mortality is not uncommon.

Fluctuations and shifts in alkalinity can cause harmful coral tissue necrosis. The link between bacteria-driven systems, alkalinity, and coral mortality is still not fully understood. However, with some good husbandry and testing practices, this can largely be avoided.

Ideal ULNS parameters are alkalinity 7-8 KH, 400-450ppm calcium and 1,250-1,350ppm magnesium.

What about commercial products?

Some products rely on organic carbon and bacteria. Few companies release the ingredients of their products, making it difficult to assess and compare these systems. That said, most of these incorporate organic carbon source, bacteria strain supplement, amino acids and trace elements.

Arguably the most controversial additives included in these proprietary ULNS systems are amino acids and trace elements. The bacteria and corals can utilise both, but do we really need them?

Testing of total nitrogen and various trace elements is not only difficult but cost prohibitive.

Anecdotal evidence suggests some validity for both. For example, only in these bacteria-driven systems are we seeing concerns with potassium levels.

Interestingly, potassium is utilised by many bacteria strains for the correct functioning of enzymes and other processes. Amino acid solutions have been noted to darken Acropora coloration, suggesting nitrogen limitation may be a concern.

No matter what product line or DIY method is chosen, each aquarium will be unique in its bacteria, carbon, nitrogen, and phosphate profiles. Product guidelines are just that and must be fine-tuned by the aquarist for optimum performance. This can be difficult and is why such systems are not advised for the beginner.  

Why are zeolites sometimes used?

Zeolites act as a molecular sieve to attract compounds or elements. They are well known for ammonia removal in freshwater and these are utilised in a similar fashion for ULNS. Most zeolites marketed for freshwater also have an affinity for calcium and are not suitable for saltwater — so be careful when making your choice.

There are more than 100 types of zeolites. Manufacturers rarely state types used, but market them as ammonia binding. The extent of ammonia adsorption is likely much less in an ionic solution (saltwater) than in a non-ionic solution (freshwater). However, it is assumed that absorbed ammonia creates a nutrient-rich layer on the zeolites that provide an ideal substrate for the bacteria to colonise.

Without knowing the specific zeolites used it is difficult to determine the water chemistry impact they have in saltwater. No independent research has been done so far.

Why are some corals in ULNS pastel in colour?

Near bleached pastel coloration of corals is often identified with ULNS. Low nutrient levels do lend themselves to these soft, bright colours, but this is often taken a step further with trace elements with known toxicity to zooxanthellae.

Low levels of copper and zinc can cause corals to expel zooxanthellae. These elements are included in some popular colour manipulating additives as well as variations of the balling method.

The inclusion of such additives is a matter of choice. They are not required for these methods to be successful and ULNS can be utilised without pastel corals.

Is ULNS for me?

ULNS is intended to replicate the nutrient poor conditions found on shallow water coral reefs. If applied properly, it can result in thriving corals while reducing nuisance algae. However, these systems are powerful, costly, and, if misused, can cause complications.

These methods are gaining acceptance, but ULNS is not for everyone. It calls for strict discipline on the part of advanced reefkeepers.

Want to see some great ULNS reef tanks?

There are two excellent examples of ULNS on the PFK website. Check out Polish reefkeeper Krzysztof Tryc's State of the Art ULNS and Belgian reefkeeper Luc Loyen's Reef tank of treasure

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