Reptile & Amphibian Forums

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.

Hello.

The betta could be killing off the mollies. Bettas are aggressive fish, and shouldn't be kept with mollies (community fish). The betta may or may not be nipping at them, he could even be chasing them and stressing them out and then they die from stress.
The soapy film is not anything to worry about, but still try to keep the surface clean. I have had this problem to and had healthy fish. (I currently have no fish because my 2 bettas died of gill disease, and I am waiting for a bigger place so I can have a 55 gallon with a Jack Dempsey).
If your one molly had dropsy, that is a symptom of a tank with bad water quality (although it is possible to have a fish with dropsy without bad water quality). Nitrates really wouldn't kill your fish off so fast, but ammonia would, and you said it's fine. Anything above 0 is bad. Ammonia is lethal. You should be changing your water 10-20% biweekly.

Once your mollies died (as well as any dead fish), they do have white "fluff" on them. That is natural, and nothing to worry about although as soon as you know your fish is dead you should remove it.

As for adding more fish I would wait a little while before adding any more. Have you checked places like liveaquaria.com for replacement filter cartridges?
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Phishie

"An ounce of prevention is worth a pound of cure."

Thanks for the insight. I had thought about the betta attacking the mollies, but I've been keeping a close eye on him - he's a very mild betta, and doesn't bother the other fish at all. If anything, one of the mollies was bothering the other two.

I've been doing periodic water changes, just changing more water when the nitrates are high. I'll probably let the tank settle a bit before adding more fish.

Other than the betta there are no apparent reasons for your mollies to be dying. Even you watch the betta he could still be aggressive during the night... although it is also possible your betta had nothing to do with it... in which case I have no reasons to give you for your mollies to die.
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Phishie

"An ounce of prevention is worth a pound of cure."