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...