Annoying when they do that, hmmm?
I like the idea of transferring your A. gassizii into a bigger aquarium. The 20G 'Tall' is a pretty small-sized tank anyway, even for Apistos. For the transfer, I would get even ANOTHER female A. gassizii so you have a 1M/3F harem, and then place them all at the same time into the larger aquarium, (A 20G 'long' may actually provide enough 'footprint' space long-term, to keep everyone happy).
Provide plenty of live or artificial plants, plus rocks and bog wood etc to break up those sight lines. And, stock with peaceful, smaller Tetras. By doing that hopefully everyone should calm down and 'sort of' get along in there.
Wow... my 4000'th post?!! Geeze, time flies when yer having fun on Cichlid-forum!