I have a 38 gallon planted tank with 1 angel fish, 3 cory cats, 5 red minor tetras, 2 platys, 1 dwarf gourami, 1 molly and 1 swordtail.
When I test the water, the ammonia and nitrites are always at zero, but the nitrates are always at 20 ppm
. In three days I did a 40 percent water change, followed by two 25 percent water changes. The nitrate level always stays exactly the same.