+1 on the dip.
I had a colony that wasn't looking very good for a few weeks. I'd moved them around, up and down, more and less flow, and finally put them in my QT, thinking I'd watch them die. I never did identify any parasite, or specific cause, but I decided that a dip couldn't hurt. Into a bucket of RO water they went. I left them in for 15 minutes, shaking them around every 5, and then stuck them back in my DT in their original spot. After a few days, I could tell they were getting better; and, now they look great!