I think the amount of girls you have in that tank is ok if it is densely covered/planted. Can you show us photos? Is the tank filtered? What I do if one girl gets nippy is I put her in "time out" - aka - a clear cup, tupperware container, or small glass tank and float her in the sorority for days, or weeks until the aggression lets up, replacing the water twice a day by dumping it into the tank and refilling.
You can try rearranging decor too. If I were you I'd thoroughly clean the tank, rearrange, and add density in plants and/or decor. Keep the girls where they can all see one another and slightly overfeed them for about 5 days before releasing them together again, or longer if you want to. Maybe wait until the other girl is better, add her floating as well, feed her a lot as well...If there's anything that will work, that will. Otherwise you might need to separate them.
The fish not being from the same breeder probably isn't the issue, although many say it does make things easier. Warm/clean water with tons of hiding spots and plants might fix things up, as well as a cool down for them. My girls are from several breeders. I have 12 girls in a 20 gallon, 2 of which are giants, and I'll be adding two more. Many would say this is overstocked because its over the 1 inch per gallon rule, but my tank is filtered well and densely planted with live plants which help to keep water clean and provide cover for them.
Another tip given to me that works great: slightly overfed girls are happy girls. Try adding variety to their diet or give everyone an extra pellet or two.
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Last edited by GreenTea; 11-19-2011 at 06:03 AM.