I noticed a tiny hole in my boys fins. When I looked back the hole was gone, but so was a chunk of his fins, he had obviously bitten them to shreds. While it may be a symptom of bad water, it may just be the start of tail biting. Keep a close eye on him and do extra water changes to be safe.
When my guy chews his tail, he destroys a chunk at a time. If you see him chasing his tail/fins in circles, that's a big warning sign he's gonna chomp them. I tried distracting him, tapping the tank, even giving him a bloodworm treat. Despite that he still tore into his tail.
Hope you can figure it out, and get him healing.