I would say:
1.Fish is cold-blooded.
– refers to fish as a food. So incorrect.
2.Fish are cold-blooded.
– refers to fish generally. Fine.
3.The fish is cold-blooded.
– refers to a particular fish, or fish in general. Okay; but it's unusual to use it in the latter sense, as "fish" doesn't relate to a genus or family.
4 The fish are cold-blooded.
– refers to several fish. Fine, but not common.
MrP