I think it would pollute the water do to the fact that dry dog and cat food usually leave a powder or crumbs where they were eaten. Just my opinion though.
Look at the ingredients of the food...I suspect it would contain lots of bits and bobs from terrestrial animals that a fish's body cant process, which would be bad for the fish's liver - same reason your not supposed to give your fish chicken or other meat too often.
I guess it would depend on the type of fish, but how often do you think your fish species would encounter animal proteins in the wild ?
Possibly OK every now and then, though I dunno why you would want to lol .