How long have you had them. Females can be agressive towards each other, it may also be possible that you have a rogue male in there. This has happened to us 3 times.
If you're sure they are all females you could try taking them out and bagging them separately, rearrange the tank and make sure there are lots of hiding places. After about an hour let them back in one at a time with the least aggressive going back in first and the most aggressive last.