It's countable if you are talking about the individual creatures:
How many fish can you see?
There are some fish in this pond.
NB the plural 'fishes' exists, but it is rarely used, except by small children.
As with all food, there are cases where a countable noun can be used as an uncountable one. For example, you can buy, cook and eat 'fish' as a general foodstuff. Just like butter, meat, milk, rice and so on are uncountable, so can 'fish' be uncountable. If you cook a large fish, you would serve it and and say
'How much fish do you want? A lot or a little?'
This is using 'fish' in an uncountable sense.