'Tá iasc ag snámh fuibh' should do the trick.
'An t-iasc' means "the fish" because 'an' is the definite article, "the". Irish doesn't have the indefinite article "a", so if you want to say "a fish" you just say 'iasc'. E.g. 'caithim gúna' means "I wear a dress" rather than "I wear dress".