Sorry that I can't advise what to do, these symptoms/video from the other fish are not enough for me to have a clue about what is going on, at this point I would say likely an illness, if tank params are ok. As you have no QT where you could start a generic treatment against the most common stuff, you are limited in what you can do (though DT being empty helps a bit and you could give various baths etc).
Imo your best bet is to post fresh tank parameters and a fresh picture/video of the fish and wait until a
#fishmedic wakes up, its nighttime in the US so you wont get an answer for some hours, then I am sure they will come and give a better opinion than mine.
I hope whatever happens it works out for the fish.