11-11-2011, 01:32 AM
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My first guess as to the culprit would be the betta. Although you have said you've never seen him acting aggressively, a lot can happen after the lights go out. Other than that, I would move to the other male guppies as the next likely aggressors. While they are supposed to be peaceful, in my experience with three different types of livebearers (guppies, mollies, and platies), I have noticed a tendency for some males to be complete jerks (other than platies, who seem to be all peaceful, all the time), especially if there aren't enough females present.