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by marcus_holmes 1137 days ago
Fish is the plural of fish only if all the fish are of the same species. If it's a mixed group of fish then the plural is fishes [0].

Same with maths - if you only have one type of mathematics then math would make sense. But there are many types of mathematics, so we call it maths.

[0] https://www.grammarly.com/blog/fish-fishes/