01-29-2011, 10:13 AM
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It is important to make sure you notice who is aggressive, as that will help if you have to take more drastic measures. For now though, just change up the decorations in the tank. Move everything around. It can be a little stressful for the fish, but then they are all in a "new" tank and it sort of evens the playing field. Are you feeding them enough? I've seen otherwise docile fish get much more aggressive when food is scarce. What are your other water parameters? (ammonia, nitrite, nitrate).
Lastly if you finally do identify the molly as the aggressive fish, and nothing else helps, you can isolate her for a while. I have a small 2.5 gallon that I use for this. I wouldn't leave them long in a tank that small, but it has a heater, filter, and air in order to keep things moving. I also print over gravel and part of my bio bag for the filter to make sure you don't get an ammonia spike! 24 to 48 hours away usually makes them much less territorial when they go back.
My warning to everyone for now though is that I'm still new to this myself, so this myself, so don't go on my word alone. I'd wait for someone with more experience to confirm what I've said.