Yes - these words have both countable and countable forms. Look at the following :
Fruit/Fish is good for you. The speakers sees the fruit/fish as a foodstuff, a mass (all fruit) and therefore singular and uncountable.
Citrus fruit / Oily fish are good for you. This time the speaker has individual types of fruit/fish in mind (oranges, lemons, grapefruit / mackerel, tuna, salmon) and therefore chooses to use the word countably.
As I said before, it's a choice and will depend how you see the object.
In the case of these two words though, the situation is slightly complicated by the fact that they also have regular (though more rarely used) plurals - fruits and fishes.
So - in one article from
Wikipedia, I found all four possibilities :
Fish ... is an important food source (Uncountable, singular)
An example of
a fish that has become endangered is...(Countable, singular)
Fish are abundant in most bodies of water. (Countable, irregular plural)
In some
fishes, capillary blood flows in the opposite direction to the water. (Countable, regular plural)
... and you could do the same for fruit :
Fruit is an important source of Vitamin C. (Uncountable, singular)
An example of
a fruit which is rarely grown nowadays is... (Countable, singular)
These fruit are in season in June and July...(Countable, irregular plural)
Citrus
fruits are grown widely in the south of Italy.(Countable, regular plural)