killer fishy?

[poopybarkflounder]

I have currently have 3 neon tetras, two cardinal tetras, two strawberry tetras, three fancy tailed guppies, two glass catfish, a mommy and baby platy, and four ghost shrimp in a 20G tank. i started out with 6 neons but three died not long after i got them and it seemed they might have been killed by another fish but i wasn't sure and i thought all of my fish to be peacefull. I originally had four fancy tails, i believe they are all males so there is the occasional chasing and a fin nip but nothing too bad. today My favorite guppy had a nip out of his fin but seemed to be getting along with the other fish so i figured he just got sassed at but nothing to worry about. wrong. just now i found him dead at the bottom of the tank with is tail fin bitten off and a his stomach bitten out.
any idea who the killer might be? I'm not sure what to do.

[RTR]

The stomach-eating is standard postmortem feeding by any or all the living fish. It was not the likely cause of death, but came afterward.

Your tank might do better with higher numbers of fewer different fish. Two or three fish is in no way a school or security for those fish. The glass cats (actually moderately long-lived fish) especially tend to fail to thrive in small numbers, but the Tetras would also do better with minimum 6-8 of a kind.

[poopybarkflounder]

is the tail eating also standard?