This had so much potential for a stomper of a punchline, but then he said "That's it!"
And used his laser eyes to kill everyone...

The backup characters made so much of a fuss calling him a "Mer" and implying he was sissy- and you also put in explanation as to why he is "Fishman".
With those elements, when he said "That's it!", The joke was there, ready to just burst forth where perhaps Fishman yells "That's it! I'm more man than any of ya'!" and motions his pants down, maybe a zipper noise- then the reaction faces of a couple men with shattered egos, and some swooning women.
But.. you took the easy way out, and killed everyone. That's cool too.

Well. Cool premise. Coulda been waaay more creative with it dude, you needed to step outside the predictable joke box.

There was nothing amazing going on, but I liked it and that's what matters.

The animation was a bit choppy, and the voicing, while good, really didn't match up to well. The sprite image explosion seemed kinda lazy and the eye beams were more "huh?" than "lol wtf".

The voice acting was pretty good, but everything else pretty much bored me. The idea is not very original either.