Hi,



I have a six-month old, 20-gallon tank, and I'm having issues with my mollies dying off at an unfortunate rate.



In a newbie mistake, I overstocked the tank at the beginning, but I was able to get it to cycle in a month or two, and ended up with four cories, three mollies (two male, one female) and a betta. The tank was fine for a few months, but now in the past month I've had all three mollies die on me.



I checked ammonia levels, and they're fine. The temperature and pH have been normal. Nitrate levels were slightly high, but not in the danger zone, after the first death, so I did a 1/4 water change, cleaned the gravel and cut down on how much I was feeding them. There is a small, almost soapy film on the surface that I can't identify, and I'm trying to get rid of.



There was no sign of disease on two of the mollies. The third obviously had dropsy, although I don't know what caused it. The two that had no symptoms were acting completely normally up until their deaths - coming up to eat when I opened the top, swimming around as normal, playing with each other.



Around the dead fish, and when I clean, there are these white tufts of something, I'm not sure what - I'm thinking it could be bacteria, but I'm not sure. I'm also wondering what would affect only the three mollies, but leave the cories and the betta alone.



Any advice? I'd like to add more fish to the tank, but I'd rather not until I can come up with a way to keep more fish from dying. I'm also wondering about aeration - I'm using a whisper in-tank filter that says that it's rated for 20-gallon tanks, but I'm not sure about it. I've also been having trouble getting ahold of new carbon filter for it - my local pet store (which sold me the thing) doesn't have them, so I've been keeping the old carbon filter in (cleaning it periodically) while using bio-bags with carbon in them for the second filter in the device.

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