Your second question:
One fish is always fish. Multiple fish of the same variety are still fish. A group of fish of different varieties are fishes (Use "fishes" only when you want to emphasize the different types of fish)
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Your second question:
One fish is always fish.
Multiple fish of the same variety are still fish.
A group of fish of different varieties are fishes (Use "fishes" only when you want to emphasize the different types of fish)