04-12-2006, 04:16 AM
Join Date: Jun 2004
In a brightly lit tank with high CO2 and adequate fertilization, nitrate is usually so low that extra nitrate has to be added to supply the plants' nitrogen demands. If you have an overcrowded, heavily fed discus tank, then maybe it wouldn't be necessary to add extra nitrate, but the nitrate should not be high.
There are a number of problems in the values that you gave us. At a pH of 6.5 with a KH of 120 ppm, you should have a CO2 concentraton of 72 ppm. 72 ppm CO2 is so high that is is probably lethal to anything that isn't gradually
acclimated to it. Even for animals that are acclimated, it is probably not a healthy condition.
It normally is not possible to maintain 72 ppm of CO2 with yeast CO2.
With 4+ watts per gallon and such high CO2 your plants should be growing like mad and the nutrient demands in your tank should be enormous. If you provide a broad spectrum of nutrients then the plant growth alone should keep nitrate levels down. Under any other conditions, your plants should be sickly from nutrient deficiency.
All-in-all, the algae succession you report sound like a low-CO2 tanks, It reads like either one or more of the numbers you gave us is incorrect or something in your water is forcing the pH low.
Something in the water that forces the pH down is likely to be a buffer. This could be something that you are adding or it could come from something in the tank, such as driftwood or peat.
Do you use tap water in the tank? If so what is the nitrate level in your tap water?