Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
Define "high nitrates". Everyone will do it differently.
At 70-140ppm nitrate (weekly range), I saw stunting and slow growth in many varieties of plants. Signs of phosphate deficiency, despite levels of 30ppm. Uncontrollable algae. The fish, oddly enough, were fine.
This due to a heavy fish load combined with EI dosing. I didn't trust my test kit either, so I bought another much more expensive one. Same result. Weighted my fish food with a gram scale, converted the nitrate/phosphate content to an equivalent amount of dry ferts based on the manufacturer's elemental breakdown, ran everything through Wet's calculator. Again, same result.
I stopped dosing the nitrate/phosphate parts of EI and now everything's peachy.
If asked for exact numbers, I say 20ppm is a good target, 60ppm nitrate is the max one should allow. Will some plants grow better at 60ppm than 20ppm? Or some worse? You'd have to experiment with each individual plant variety. But 20ppm will grow any plant adequately.
Some people even have better plant growth if they keep nitrates well below 20ppm, and other macros similarly reduced. These people typically have soft water, and there's some speculation of a connection.
Experiment and see what works best for you, in your own tank.