When you're a fish, the selection of kitchen gadgets available to you is somewhat limited. You've pretty much got a choice between rocks, and nothing else. For the first time, a fish has been caught on film using a rock as a tool to open up a clam, making it a member of the exclusive club of tool-using animals.
Primates (humans included) are well-known tool users. It's probably not surprising that other "intelligent" animals also use tools, like elephants, dolphins and crows. Both birds and sea otters use rocks as tools to gain access to food that would be otherwise locked away, and now fish have been spotted doing effectively the same thing to get the tasty insides out of a clam.
The fish in question is a blackspot tuskfish, which a diver in Australia documented picking up a clam and repeatedly bashing it against a pointy rock to break it open. The fish was apparently "landing absolutely pinpoint blows," and shell fragments around the spot suggest that not only was this not the first time the fish had tried this, but that it consistently kept coming back to the same rock.
Some scientists have expressed skepticism as to whether or not this actually constitutes tool use, generally because the definition of what tool use is was originally designed to be applied to primates and other animals with hands. For example, birds that drop bones on rocks arguably aren't tool users, but birds dropping rocks on bones probably are, the salient difference being that the bird has to take an extra logical step when it drops the food and picks up the rock.
On the other hand, fish don't really have that option, and it just makes more sense for the fish to just do what it's doing rather than try and pick up a rock to bang against the clam. So the only thing that's really for sure here is that our piscine pals seem to be quite a bit smarter than we give them credit for.