Female bettas biting each other

Question
I have a ten gallon tank with a heater and filter. I recently changed the filter. There are four female bettas and a plecostomus in the ten gallon tank. I've had the four girls for a little over two month and just added the plecostomus a week ago. The bettas are flaring their fins and biting each other. They did this when I fist got them, but stopped a long time ago. Why are they fighting each other now?

Answer
Hi Peter,
I know they say that female Bettas may be put together in a tank, but the tank should be big enough so that they don't run into each other all the time.  I personally don't recommend putting more that one female in a ten gallon tank.  They are Bettas, and are just has bad as the males as far as I'm concerned.  They are aggressive, and will pick at each other.  The weaker ones will get stressed, then sick, and then death if you do not separate them.  Maybe your Pet Store would give you a credit for the other three fish.  Four Bettas is too many in a 10 gallon tank.  The strongest one, will eventually kill the weaker ones.  They are not peaceful fish, when put together.
You ask why they started fighting each other now...well I don't think they ever really stopped.  It might have been less noticeable, but when the strongest one got the upper hand, now it shows more.  I hope you pet stores gives you a credit, and that you will separate them, as this behavior won't stop, it will only get worse.  I'm sorry to bring you this bad news.
Lynda