Why is it "this is a fish" rather than "this fish"?
Seo means this is, and iasg is a fish. However, seo can also mean here, so if you wanted to say 'this fish', you would say 'an iasg seo'. Literally translated that would be 'the fish here' i.e. this fish.
(Is*) seo iasg = this is a fish
an t-iasg seo = this fish
*Is = the copula 'be'. It's notionally there, it just disappears in this kind of "this/that is…" sentence.
I typed "this is fish" as in a food, and it accepted it.