I'd guess the nippers were the Serpae tetras. The 20X tank is around the size of a 10 in terms of surface area, and I don't really think it's a good idea to house that many fish in there.
If you want to get old-school, by the inch-per-gallon rule (which can be used sometimes as a rough estimate, as long as you estimate by the ADULT size of the fish,) you've got:
10 serpaes: 2.5" each at adult size, so 25"
5 guppies: 2", so 10"
3 swords: 5", so 15"
2 angels: 6", so 12" (extra height needed because they get very "tall)
2 spotted cories: 2.5", so 5"
25+10+15+12+5=67 inches. That's a lot of fish!
Also, cories need schools, so you need at least 4 more of them for them to be happy and thrive.
We're not trying to bash your skills, just trying to get your fish into a good home
Welcome to the Forum, I hope you get lots of help here and can help contribute!