ick

Ich is a paracite that unluckily we can only visually see the final stage. Damsels which are very hardy to get it is a sign of stress to the fish. I personnally would treat in a qt using hyposalinity and keep out of the display tank for 30 days to make sure that the rest of 3 stages of it are gone.

Its called stress, or improper acclimation or any number of things. There is a debate that the parasite resides in all tanks and on all fish. I would debate that if people would aggressively QT their fish like they are supposed to.

You brought the fish home, it got stressed, it has ick.. Now you'll need to QT or the rest of your livestock will get it if they don't already.

What kind of fish? Size of tank?

It is very common for new arrivals to just "get" ich. It's either in the tank to begin with or is on the fish already and during the move from the LFS to your tank, the fish get's stressed to the point where the fish becomes vulnerable.

Do not run out a get some miracle ich cure.

You can:

1. Let the fish try to live through it.
2. Treat with copper sulfate in QT tank
3. Treat with hyposalinity in Qt tank at 1.009-1.010 SG (most desirable) and leave display fishless for 4-6 weeks and QT all future fish in hypo.

Assume every fish you buy either is infected or is carrying ich.

The ich parasite got in the tank most likely from the damsels themselves. There is no way for it get in your tank unless you put it in via an infected fish or on substrate that is brought in from a tank with an infected fish. I agree with the advice to treat the damsels in a qt or hospital tank. However I'd leave the display fallow (fishless) for longer than 30 days, I'd leave it empty for 6-8 weeks.