I imagine the "small fish" is meaning the term which usually refers to a weakling or easy target; small fish are easy to catch, but big fish put up more of a fight.
The "small fish" thing (雑魚, zako) is a phrase meaning easily-killed monsters or "cannon fodder" troops in games.
There's a reasonably good explanation for how this came about in the Wikipedia entry on Zaku (the Gundam mecha): https://en.wikipedia.org/wiki/Zaku#Concept_and_Development
@freakofnature@Foreverhound wouldn't it be better to use the English term with the exact same meaning, "Small Fry"? Fry in this context are another word for little fish.
@Fushiginiku Not sure what you mean? 'Small fish' is a perfectly reasonable English phrase. It's not used as commonly as 'small fry' but that doesn't mean it's a poor phrase to use.
@Foreverhound must not be used in my part of the world. I've heard 'small fry' all my life with this meaning, but never 'small fish'. Although I've certainly heard the opposite, 'big fish'
@foreverhound I've actually only ever heard "small fry", not "small fish". I could see how that might be used in some areas as a colloquial version, though.
@Fushiginiku is right, "small fry" is what you usually use in English, to describe trivial enemies, barely worth making an effort to deal with. The meaning is the same as "small fish", but it's common usage to say "small fry".