Fin and tail rot is a disease, so like most diseases strikes when conditions are bad or fish are stressed. Any new fish that could have been carriers? Low pH can also erode fins. But once a fish is down, boulie is right, even 'peaceful community fish' may join in the nipping. Watch carefully the tails of the rest of the fish in the tank. One ragged tail is nipping, everybody with ragged fins is disease.
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