I don't know much about this so I am not sure anyone can help me. I got 4 new neon tetras and 4 corys. I put them in my 15 gallon along with one black molly and 2 neons I already had. In 2 days 4 neons have died. I saw no signs of disease on them except the last one that died yesterday. It had cotton like stuff on its head. Then today I wake up and one of the corys is dead. It had some streaks that look like blood on it and it tail was missing. I don't know if some of the others tried to eat it or what. I will be real upset if all my fish die so I need to find out what I can do to stop this. Thanks!
Ammonia 0
Nitrite 0
Nitrate 10
Ammonia 0
Nitrite 0
Nitrate 10