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I have never personally kept more than one female betta in a tank, but have heard about it being done quite successfully. There is never a guarantee that they won't fight, or that one of them won't be killed, but there is a better chance of them getting along in groups. If kept in pairs, one female will assert her dominance and might be extremely aggressive towards the other fish; this type of set-up can be just as bad as two male bettas living together. If you are going to try to put several (3+) female bettas in one tank, you will 1) need a bigger tank (at least 10g), and 2) make sure there are plenty of plants (real or fake) and hiding places. It helps to have plants because they break up the line of sight so the fish don't have to see each other all the time.

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When in doubt, do a water change.