I went to the local fish store last night, signed up my kids on a fish club and they received two new Silver Tip Tetras for free. I currently have three Dalmatian Mollies, and Female Betta. I put them in last night and they seem to be getting along with the current fish. I understand that tetras are schooling fish and that I should follow the inch per gallon rule. I have a 20 Gallon Long tank. How many more of these tetras should I add? I plan to get an algae eater, most likely an Oto Cat in the upcoming weeks. Any recommendations will help.