Hi, We are a newbie family who just got a small (5 gallon) tank in August. We started out with what I now know was too many fish - 3 platys and an albino catfish. We don't have live plants, I use an air filter (not the smallest at petsmart, the model for a larger tank than ours, a "20'). I change about 20% of the water each week and use test strips each week as well, although I will be switching to a liquid test. One platy died about 3 weeks in, the second died of what we also know now was ammonia poisoning due to overfeeding, about 3 months in. I did a major water change and the nitrite/nitrate levels have been 0 for the last two months. We switched to once-a-day feeding at that time as well. The PH was at about 6.5 but it's now 7, after I used API Proper PH. Two days ago our last platy, a Red Wag, began to lie on her side (I am pretty sure it was actually a male but my daughters called her "she" and gave her a girl's name) and rest near the wall of the tank. She was still moving around a bit but by this afternoon she pretty much stopped moving. I was able to see that her gills seems very white and possibly a bit puffy, and she was gasping for air. Her eyes seems to be popping a bit as well and she did seem a little thin, but she hadn't eaten much for 2 days. We also noticed that some of the scales near her tail seemed to have flaked off. However, her bowel movements were normal brownish and she didn't have any other white spots. About an hour ago she spun around a few times and lay down, and then she died. The catfish is still her usual self. I want very badly to get some more platys but I am worried that I did something wrong that killed them. I would actually only get one, since that will pretty much be the max for this size tank. I am wondering if I should move the catfish to another location while I completely clean out this tank, although I hate to start all over when our water seems to be in such great shape. But I am worried that our platy died of a bacterial condition that could infect the catfish. Any advice would be very welcome. We really miss our platys.