I have a freshwater 20 long .It is cycled with 2 filtera running. I have 2 air stones, Anubis, sand and other decorations. I have lost quite a few fish in this whole process. We start one yr ago and upgraded to the 20 over the summer. I just lost another guppy. I do weekly water changed of 25-50% (some weeks smaller, other weeks larger). When I tstd the water Tues durodua changed it was 7.8 pH, 0 ammonia, 0 nitrites, 40 nitrates. I recently noticed the nitrates are high than they used to be even though I didn't change anything I was doing. What would cause this and is it harming the fish? Is there anything else I am not doing? I do have the same 6 glofish danios from when we switched to a 20 and a platy that was the only baby who survived. I'm not sure why they are fine and yet I keep losing other fish. (Platies, cories and guppies to be exact). Thank you.