I set up a sorority tank last week and everything has been going great. All the girls are happy and healthy. I exchanged one of the original ones I got the other day because she was constantly causing problem with the others. Well, after two days of what seemed fine, the one I replaced her with was dead this morning. The other fish appear fine, they all ate this morning, and everything. What should I do? I removed the dead one, but now I don't know if I should do a water change. Please help. Thank you.