It's a male goodeid, probably a butterfly goodeid Ameca splendens. This family of Mexican fishes are only distantly related to the common ovoviviparous livebearers like guppies and swordtails: they are truly viviparous.
I was going to say Ameca splendens, but it's many years since I kept them -- had a good breeding stock back in the eighties, when there weren't many about.
