If a fish has fin rot they won't open the fin it will be like it's glued down( easiest way to explain it) they normally will lie at the bottom and not move very much if at all, I've lost a few fish to this as they had it when I brought then from pet shop, this looks more like nipping or it's caught it on something, also the tail fin will rot away as well, I haven't got pics to show you what it looks like as I don't like taking pics of fish with fin rot, hope that helps a little one last thing most fish with fin rot won't eat either
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