What level of nitrate is toxic?

[saltydog]

I have no plants in my tanks. (Lighting levels are low & some of my  fish do not seem to toloerate them.) I do fairly large water changes (40-50%) only every 4-6 weeks. Nitrate levels test at 80-100 ppm (the upper limit of my test kit, so likely higher) even AFTER most water changes.  African & South American cichlids breed in my tanks & my fish generally seem healthy with good colour, appetites, & growth rates. Perhaps maintaining nitrate levels in the 20 ppm range as is recommended in test kit literature is unnecessary (except for corals in marine tanks).  One aquarist with a  background in water chemistry told me that the main reason for doing water changes is to eliminate DOC's (dissolve organic compounds). Comments anyone?

[Ron]

Hi Saltydog,

Nitrate sensitivety varies among species of fish. It seems to be generally held that most tropical freshwater fish will have no problems with nitrates if the levels are kept below 40 pp. In doing nutrient dosing experiments in planted tanks, I have had nitrates in excess of 80 ppm with no problems among a fair cross section of commonly kept species.

On the other hand, people keeping freshwater stingrays have reported problems with nitrate levels over 5 ppm.

In my planted tanks, I shoot to keep nitrate levels between 10 and 20 ppm. I stock my tanks quite heavily, and do 50% water changes once a week, after which I have to add nitrates to keep the levels up where I want them. My primary reason for the large water changes is to prevent the buildup of dissolved organic compounds, which I have no means of measuring. Others have good luck doing much smaller, and less frequent changes, but this works well for me.

I honestly believe the frequent large water changes make it possible for me to have bred many species of fish in my 12 kH hardness water that supposedly need softer water to spawn successfully. It makes the most notable difference in tanks being used to grow out fry. I especially get a kick out of spawing various tetra species in my "hard water".

Best,  

Ron