Did the gravel ever get vacuum, cause you mentioned that your dad over feeding the other fish, maybe bacteria formed in between the gravel.
hmm. That's an idea too. I know he did gravel vac on occasion. And I know I did. The good news is that the gravel is gone! However, maybe I should empty it completely....do a deep clean of everything, the tank, lids, lights, filters, etc, re-seal it, bleach it. Start over new. before I put anything in it again. It's just been running with water in it and the decorations. So now would be a good time lol. Maybe I just want another project so I can focus on fish stuff instead of doing homework.