Fish are often plagued by external parasites, which are presumably difficult for them to remove.
There are well known cases where large fish come to coral reefs and allow smaller fish to pick off their parasites. See below a moray eel being "cleaned" by a shrimp.
Some fish, like sunfish, even get cleaned up by birds: https://www.youtube.com/watch?v=U60obmWODLQ
But on land, we have many social animals that solve this problem more directly by grooming each other. How come the fish never seem to do this? Many of them are already social, and I imagine they wouldn't have much trouble digesting the external parasites. Why don't they groom each other?