Which fish swims the fastest?
No it is not the swordfish but the sailfish.
In the example sentence, "Which fish in the aquarium swims the fastest?", the noun "fish" may be singular or plural. The noun "aquarium" is singular. The noun "fish" is an uncountable (mass) noun as a word for a food substance, but can be used as a count noun (fish, fishes) as a word for the creature (creatures). Example: How many fish are in your aquarium? Or: How many fishes are in your aquarium? (both sentences…