I always had a suspicion it was the plastic plants in his tank that were ripping my Sherman's fins but I never had any solid proof until this morning when I noticed a rather sizeable piece wafting on the current, firmly stuck to one of the more spiky plants - like cloth stuck to a tree branch. It appears to be some kind of plastic hornwort or something similar. So now I know - bettas and spiky plastic do not mix. I can't take them out just yet but Sherman will have all fabric/silk or live plants in his new 3g tank. And a friendly warning to all: if your bettas fins are ripping and you have plastic plants, remove the plants before accusing your fish of being a tailbiter.