Translation:These fish swim in seas and rivers too.
Is my answer so wrong? These fishes swim in the seas and rivers too. And: Is "these fish swim" correct?
:-) Thank you! What i don't understand is not the plural or singular use of fish or fishes, but the mixture. I miss the S of swimS if it is only fish. What i learned as a not native is: This fish swimS - for singular and plural and These fishes swim? Is there a special rule in this case? And the other thing is, that the plural was marked wrong. I thought, both versions are correct in plural. (fish and fishes)
Fish as a plural is treated the same as any other plural.
This fish swims (one fish only)
These fish swim
These fishes swim (multiple species of fish; not as commonly used as the above but you can report if it doesn't accept it)