I think they are a type of syno, not sure which kind. As soon as I saw the picture I thought, "Another person got suckered into them." I got a syno eupterus when I was young and naive. The guy who sold it to me said they get 3.5" tops. After I got home with him I realized that he would grow up to be 12"+ easily. They are loud (take them out of water and you'll see what I mean) and rambunctious fish. The one I had constantly uprooted plants and knocked my rock work over. It went from a 10g, to a 30g, to a 55g and is now in a 120g cichlid tank.
I would recommend re-homing them especially since you will not have a 55g till December. They will be way to large for your 20g by then. Even then, they will probably be too large for your 55g.
Here is a pic of my upside down catfish:
And my syno:
You can see what I mean by not thinking it is a eupterus. Yours look like they are missing the distinctive 'featherfin'.