Quote Originally Posted by Culfinrandir View Post
A slightly wrong English Teacher actually. When fish are of the same species, eg mackerel, then you would say you had a tank of fish. If the fishes are of various species then you would have a tank of fishes.

It is perfectly normal & even preferred to use fishes as the plural unless you know for certain that all are the same species/type.
I stand corrected and apologise embarrassingly.... I blame my English teachers when I was at school >.> Maybe it's an Australian English thing...