Originally Posted by jodymaze
I think nitrate and nitrite have me confused...i may have typed the wrong word ..i will look at my test kit tonite when i get home and repost.( no fish died yesterday)
Ammonia and nitrite (with the "i") should be zero once the tank is cycled, and remain zero. Nitrate (with the "a") may vary depending upon several factors, but the lower the better. Most of us recommend below 20ppm. Plants should achieve this. And partial water changes weekly.
Responding to your earlier question on water changes. If the aquarium is biologically balanced (fish load, plants, water volume, not overfeeding, etc), a weekly change of 30-40% of the tank volume is advisable. Fewer fish and more plants allow one to change less water, but it does no harm to stay at 30-40%. I do 50-60% weekly in all my tanks, they are heavily planted but I have a lot of fish in them too. I never have nitrate above 5ppm (due to the plants).
Water changes remove stuff that no filter can. Live plants handle some of this, but this is only adequate without water changes if the fish load is moderate and plant load is very heavy. Provided the parameters are identical or very close, there is no harm in water changes. The more fish in the tank, the more these are needed. With fry, you want to ensure they develop properly, and that means either much larger tanks or more frequent water changes. Discus breeders raise 40 fry in a 20g tank, but they change 90% of the water 2, 3 or more times each day. Otherwise the fish would be stunted. Fresh water is essential to the proper development of fry.