I have a 20 gallon aquarium with 2 angelfish, 2 gold dust mollies, 2 black mollies, 2 guppies & 1 red tailed black shark. I setup the tank september 30th of this year. I let it run without fish for about 1 week. I checked ph, ammonia, nitrite & nitrate. Ph was slightly alkaline 7.2ish and the other 3 tests read 0 ppm. Anyway the fish are swimming around and eating with no visible signs of issues. I have a bio wheel hang on filter which is working great and I change filter cartridge when it starts to overflow over top, I also have plenty of aeration (an air stone on one side of tank and a volcano on other)The temperature is 78f all the time. I don't over feed and do weekly 25% water changes. My ph & nitrate read 7.0(ph) & nitrate is 0ppm. My ammonia is .25ppm & nitrite is also .25 ppm. I am baffled on whats causing this. I purchased my fish in october & early November on separate occasions(at least a week apart). Except the one time where I bought: male guppy, 2 black mollies & red tailed black shark the same time. Am I in danger with my water quality of losing fish? What should I do?