Shoulda finished High School

OP, when someone won't listen it often helps to go to an outside expert. If (when) something like this comes up again, show her the entry and explanation in a dictionary and/or grammar or usage guide.

As I understand it, "fish" is similar to "money" and "deer" in that the plural forms are "fish", "deer" and "money" if they're all the same type, but "fishes", "deers" and "monies" if they're different types, or from different sources. Anybody understand it the same way?

Both are okay, OP! I prefer "fish" too. But even my college biology book uses "fishes."

The plural of fish has 2 possibilities. Fish or fishes. Just like the plural f cactus could be cacti or cactuses.

And, it seems, is a better grammarian than you are OP! Don't hold your breath for a raise! YDI for having a crappy attitude.

Technically, the plural of fish is fish unless there are different kind of fish in the group in which that case it is fishes.

Fish is the plural for multiples if the same species of fish. Fishes is the plural for multiple species of fish.

I think we need more evidence to see just how much of an idiot the person is.

Ask her what she thinks a School of Fish is. That's how I learned my plural words....when I was a kid.