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)