I do have an idea.. I look for a picture but this is what I think of right off the top of my head.
Isopod Parasites (very rare):
Symptoms - These are more frequently seen on shrimp, but some species target fish. They are nocturnal and difficult to distinguish from beneficial amphipods you may have in your aquarium. So, the best way to detect their presence is to watch your fish for evidence of their existence. You will notice bite marks (it’s most evident on any yellow fish), or even the alarming sight of some attached to your fish when the lights first come on. There are even “tongue eating isopods.”
Treatment options - First, remove all fish (except one easy-to-catch yellow fish). The yellow fish will be used as “bait” to draw out and remove the isopods from the fish’s skin (by using tweezers). Rinse & repeat until there is no more evidence of fish-attacking isopods in your tank.
