I need help. I started a 15 gallon tank 3 months ago with 1 pleco, 1 Platy and 3 mollies. The mollies all died within a week. The platy and pleco were still going strong so we decided to try to add more fish. Added 2 more mollies about 2 weeks ago, both gone. Added 1 more platy, a guppy and another molly last Friday. 2 days later the platy seems ok but the guppy and mollies are gone. What am I doing wrong???