Do you capitalize the names of fish?

Capitalisation of fish names depends on the context.

If you are talking about the name of a specific fish, for example a pet fish, you would capitalise (eg. "Bob" my goldfish)

If you are using the common name of a type of fish in a sentence, you do not need to capitalise (eg. I ate salmon/trout/sardines last night) unless it is the start of a sentence.

If you are using the scientific name, you capitalise the genus name but not the species name (eg. Salmo salar)