Are Koi Wusses? Part VIII - Nitrate

Nitrate is the product of  the oxidation of Nitrite by, primarily, the bacteria genus Nitrobacter, and, to a lesser extent, the genus Nitrospira.

It is generally believed that Nitrate is toxic to Carp (Koi) only at very high levels. There is much disagreement as to at what level does Nitrate begin to have toxic effects. This is primarily due to the lack of research done on this subject as relates to Carp (Koi). Some believe that most fish easily tolerate levels below 100 mg/L; others claim that levels above 120 mg/L are toxic; still others state that fish can tolerate levels up to 500 mg/L.

The important aspect of Nitrate toxicity to remember is that it is species specific. Scientific research has shown that Nitrate levels of 10 mg/L, which is the maximum safe level for drinking water, IS toxic, at least in the long term, to certain sensitive freshwater invertebrates, fish and amphibians.

The toxicity level in an eco-system pond is, for all practical purposes, a non-issue.

Aquatic plants utilize Nitrate as a food source. If excessive Nitrate occurs, Nature has a built in balancer…ALGAE. We know that an abundance of algae (Pea Soup) is symptomatic of more basic problems such as overfeeding, overcrowding and accumulation of organic debris, among other things and corrective steps should be taken. On the flip side of the coin, some level of Nitrate SHOULD be detectable. A complete absence of Nitrate indicates water quality issues and could be more problematic than an excessive amount.

 (NOTE: Algaecides are not recommended as their use will very likely result in reduced Oxygen levels while doing nothing to reduce the nutrient level in the water. In fact, as the algae die and decompose, nutrients are released back into the water bringing the process full circle.)

I find it hard to imagine that Carp (Koi), given their native habitat, their preference for muddy, nutrient rich water and their adaptability, would ever have a problem with Nitrate toxicity…. at least in a Garden Pond (eco-system) setting as long as Nitrate levels increased over time and the increase was not large and abrupt.

The true Koi Pond, because of its lack of plants, does present problems with Nitrate levels. Not because it is a health problem (albeit remote) for the fish, but because of the potential need for algae control. A true Koi Pond is constructed specifically for the viewing of its occupants, so water clarity is of prime concern. Excessive Nitrate levels in Koi Ponds are usually controlled by water changes and by the process of Denitrifcation.

This will be the subject of our next post.