What an odd question! I think that it would be simpler to give similarities, rather than differences! Size isn't really a distinction - the size range of fish includes the size range of dogs. Fish have scales, dogs have hair (generally!) Fish are generally cold blooded, dogs are warm. Dogs breathe air, fish breathe water (again, generally).

Morphology is a word you could use.

Well, dogs are mammals that (in most cases) walk on four legs. Fish do not have any legs or paws for them to move around, but they do have fins. Dogs usually have fur that can also be easily styled in some cases, kind of like how humans style their own hair. Fish, on the other hand, have scales and you can't style them like you can win fur, you know? Dogs have noses, not gills, and their nose is one of their most important senses that is used. Fish have gills which they use to breathe, and I don't honestly know if fish can smell or not!

well...they have different structure? like dogs have two ears,but fishes don't have them...at least you can't see them...fishes have gills but dogs don't have them... and they breed in different ways.