There's not enough information to determine anything.
You need to go by their adult size, not their size now. What kind fish are they? Common plecos get more than a foot long, so he'll outgrow a 20g easily.
I'd assume that they're 13 inch and a half fish that aren't fullgrown, so you're probably heavily overstocked. 20 gallon aquariums don't have much stocking room.
A 30g filter on a 20g isn't much filtration. It's the minimum for normal stocking (like a small school of neons or guppies).
And I assume the tank is already cycled? If not, even a few fish is way too many because your ammonia spike will kill or, at best, stunt them.