Q: Fishes called muscles とはどういう意味ですか?
A: The word “mussel” is pronounced the same as muscle. Mussels are shellfish, kind of like clams. The shells are black or brown.


Q: Fishes have a lot of hairline stripe in its body. この表現は自然ですか?
A: A better way to say this would be: "Fish have a lot of thin stripes on their bodies." The plural of "fish" is "fish," not "fishes." The rest of the sentence also needs to be plural. "Hairline" is not the correct word here ("thin" sounds better). Physical features of a thing are "on" it rather than "in" it.
Q: Fishes are good for your health.
A fish is good for your health.

I'm writing an essay on Fish. Which one can I write?
A: You could also say "eating fish is good for your health" in this case -"fish" is the general noun, not exactly either plural or singular
Q: Fishes consume oxygen by their gills. この表現は自然ですか?
A: "Fish consume oxygen through* their gills."

"Fish" is an uncountable noun and therefore has no plural form.

"Through" is a more appropriate preposition than "by" in this context.