Freshwater dips are definately frightening.

Although I agree with Erik on the UV, I think the quarantine might be too-stressing.

I have only had one real success in quarantine; saltwater or freshwater in many years after an icky situation.

Go easy on the water changes.(although definately you need to change 40%)
go slow over a few days)

Something stressed them!

Also get 2 cleaner shrimp immediatly.
I am not an expert, this just works for me.