I run UV 24/7 on all three tanks. 2 are 300gal. But I have ick in my fish only 300gal. turning it off to do copper. The UV worked well in my reefs though.I dont like using cupper because the immune system of the fishes will go down quite a bit.
I would use 60 watt UVC or a little more and the problem will disappear.
The UVC units need correct flow and a good part of it needs to be where the fishes sleep. The parasites is spreading in the night when the fishes sleep.
They are very poor swimmers (up to 2 cm/second) so circulation makes it harder for them to attach.
Dont reduce tank flow in the night. If You usually do that - quit.
Leave the UVC on for 2 weeks after last spot sighted and Your fishes will be fine.
After the UVC use the fishes have their immune system intact. (It is the reason that the other fishes are not infected)
And I know it is doubtful if we can say it is the immune system that is working but we definitly know - healthy fishes is not affect as much as fishes in bad condition.