Classroom fish tank Tank: 60 liter (16 gallons) 3 weeks old, Filtered, 28 degrees C Water change: every 2-3 days with water that has sat for the same amount of time to reduce chlorine Fish: guppies 6 platies 6 cory cats 3 neon tetras 3 white mollys 2 + 3 introduced black mollys 2 (died) 1 baby molly (I didn't know mom was pregnant when I bought her and i was only able to save one) The fish were all happy. until my black mollies died. One wedged herself behind the breeding box on the weekend and the other developed white fuzz or something then died a few days later. both tended to hide behind the sponge filter. I would have to tap the glass to scare them out for feeding time. However, the one that developed the fuzz became more active after becoming sick. After they died I got 3 new white mollies because my other two were doing so well, and my students liked to watch them the most. The next day one new molly was dead and two guppies with it. Then during an 8 hour day, 3 more guppies and the two healthy original white mollies died. The cory cats and the neon tetras seem to be unaffected. the next day no new fish died, but one guppy and one platy seems sluggish. Both mollies have little appetite. The baby molly is in a breeding box alone and seems healthy. I went back to the store and she said hers didn't die and claimed that it was on my end. I have been taking very good care of my fish and had no fish die for weeks. I live in Korea, and I cannot find any water test kits here. However, judging how the tetras and corys seem immune, I think It must be a disease the new mollys brought. I have a second tank with semi-aggressive species and all are healthy. 1 chinese algae eater 5 albino tiger barbs (they look like nemo so I had to get them) 3 black skirt tetras 2 red eye tetras Can anyone shed some light on what may have happened? The sick molly was sick for at least 4 days while all my fish became sick and died within hours. they suddenly become lethargic and sit on the bottom, then start darting around randomly, then settle down again and die.