Thanks everyone! I really appreciate the tips!

It's funny that you mention the angels pairing up - that is exactly what has happened in the past few days. One of our angels actually died :( He started floating upside down and swimming sideways, then eventually just sat between a rock and the glass, so my boyfriend euthanized him :( It was awful, and I feel terrible. I can only imagine three things that could have happened: one) I injured him when I dropped a rock while rearranging the plants a bit. An angel "bit" my arm and it startled me and I dropped the rock (it actually hurt!) :/ I didn't see what happened, I just remember a fish freaking out when I dropped it; two) another angel, one of the pairing up ones, injured his swimming bladder (is this possible?); or three) he got sick somehow, but he really didn't look sick on the surface.

But anyway, yeah, the two pairing angels have started charging the other fish and are keeping them out of a little corner of the tank where they have started cleaning some anubius leaves. The third angel has been ostracized and is keeping away from the other two. We have a very small apartment but we could swap the 50g with the 20g once we get the glass replaced - do I understand correctly that that is what you've recommended, talldutchie? We are planning to move soonish, so then we could feasibly have both of them, but not right now. I really like seeing them preparing the leaves, but they're bullies and are really charging the other fish! The 50g tank has the same lid size so we can do it - can any of you please share some tips on swapping them?

Thanks for your patience and input, everyone. I am new to this, as I said, but I am already really enjoying it (except the fish dying part :( ). When we build the big aquarium, I want to get a nice piece of wood for it :)

Thanks again!