Thank you now I get itI agree with King'sBastion and PaulQ.
Fish are easy to look after as pets.
How many fish are there in your tank?
You are talking about a number of live fish, so it is a count noun. You could talk about one fish, as in PaulQ's post, or many fish.
Fish is easy to cook.
How much fish do you need for six people?
Here "fish" is a food, i.e. a noncount noun. The fish could be measured by weight.