It’s correct and natural either way. “Some” can be used with plural countable nouns (“some birds”) or singular mass nouns (“some water”). It can also be used with a singular countable noun, although the meaning is a bit different (“some dog”).
“Fish” can be countable (if we’re talking about the individual fish) or a mass noun (if we’re talking about the meat). For live fish, it would be countable, but for fish at the market, either way would be fine.
“Fish” is irregular in that the plural has the same exact form as the singular: “Fish”. For this reason, we can interpret “some fish” as either singular or plural — and if singular, as either a countable noun or a mass noun.