by Erik L. Johnson, D.V.M.
What if there was an almost ubiquitous compound
in our fishes' water that could stunt growth? What if there was a
compound that could slow wound healing to the point where you become sure
your antibiotic therapy is not working? What's the next parameter to check
just when you're sure your "water's just fine" but none of your small fish
ever live more than a few weeks?
Defined: What are Nitrates?
Fish waste -> Ammonia -> Nitrite -> Nitrate.
Nitrates are the final product in the successful reduction of nitrogen
Algae and plants in the presence of phosphates normally use/eliminate nitrate.
Causes: When are nitrates a problem?
Symptoms of chronic high level nitrates
In heavily stocked systems which are also heavily fed, the cycle can produce
prodigious amounts of Nitrate. The better your filtration, the more Nitrates
can be produced as you feed more and more heavily.
In such systems, the growth of brown algae is not sufficient to measurably
decrease nitrAte levels. A thick blanket of green sessile algae will measurably
decrease nitrate levels. Dense plant growth of the more ornamental varieties
such as Hyacinth and Pistia [Water Lettuce] can also assist in the reduction
When Nitrates get very concentrated, they may support the growth of Blue
Green algaes which can contribute to certain intoxications and algoses.
[Author's note: These blue green algaes are actually pale blue like blue
High nitrate accumulations are more toxic under conditions of low oxygen
tensions. Part of this is because the Nitrates can do damage to the fishes
red blood cells and to their vascular performance, causing them to be oxygen
deprived under otherwise survivable conditions of low oxygen tension. The
slightest decrease in the oxygen carrying capacity of the water, or an
increassed need for oxygen can cause sudden death.
New, small fish added to your collection die off within 2-3 weeks.
Lethargy, lack of energy, especially when you're not feeding them.
They may brighten up considerably during feeding times only to become dull
Dramatically increased susceptibility to disease
Very delayed wound healing
Redness in the fins and occasionally patches of redness in the body
Upon close inspection, fancy goldfish may have dilated blood vessels at
the base of the tail.
Large fish are much less susceptible than smaller fish.
Sudden death with the very slightest of stresses [catching, handling, injection].
Numbers over 100mg/L are associated with the signs and symptoms I am mentioning
Methods of detection:
Aquarium Pharmaceuticals has a new test kit which does not require test
Tetra Labs test kit is the one with which I am more familiar and is very
Special notes concerning high Nitrate levels:
Lack of energy is the single most common sign of Nitrate accumulation.
Short bodied goldfish consistently "list" to one side and hang motionless
in the water when nitrates accumulate.
Fish are noticeably improved with a 50+% water change, duly dechlorinated.
NitrAte accumulation is NOT an acute intoxication.
It will NOT happen that someone says: "My NitrAtes spiked and all my fish
were lost." In fact, fish must be exposed to high nitrates accumulations
for days (new fish additions) to weeks (smaller fish which have adapted)
before a symptom is noted.
Adaptation is possible under conditions of high Nitrate accumulation. Large
adult fish may tolerate levels up to 200 mg/L for long periods [months]
whereas the health of fish under 10 inches will deteriorate within weeks.
Treatment: Nitrate Control:
A major water change is the very first, best step to controlling Nitrates.
Water changes may be required daily until numbers under 20 mg/L are accomplished.
Increases in lighting will stimulate green algal growth in the recently
Algae will consume NitrAtes at a prodigious rate.
Cleaning the pond bottom of brown algal growth and removal of the filth-holding
slime in the pond is advised, but the pond needn't be "sterilized" or impeccably
Plants may be employed. In order to force the plants to use waterborne
nitrates and phosphates, a soil-less potting media may be used so the plants
can send out viable root systems but all their nutrition is derived from
Because feeding and stocking rates influence this so dramatically, a reduction
in stocking density and feeding rates can make an improvement in
the accumulation of Nitrates.
Floating plants with complete root systems can be grown in floating plant
habitats, or connected [but isolated] ponds. More
on this in another article.
Erik L. Johnson, D.V.M.
Find out more at Dr. Johnson's Web