Well this is a big debate in my area . It really depends on who you ask .I have asked people who have both salt and fresh, and people who have just fresh water. I actually took a salt water class not to long ago and the instructor was a long time fish keeper from fresh to coral. He told me the nitrates don't really matter much in a freshwater tank. I have asked on other forums about nitrate levels and have gotten a general response of lower then 30ppm. Now I have had problems with nitrates getting over 30ppm in a weeks period and have done 50% wc to make sure they stay under 30ppm. I would say 30ppm is the max but that would be different depending on the fish. My findings on the whole thing is this. We do wcs to keep the nitrates down for a reason if we did not do wcs I'm pretty sure after time the fish would suffer and the water quality would drop and turn green.