I have a 20 gallon high tank.Last week, I HAD 2 small mollies, 3 small swordtails, 6 neons, 2 small danios, and about 10 guppies. I also have a 4 inch long pleco. I have had this fish tank for 7 years. I recently had just a pair of convicts in it with the pleco. The big male died one day...