I'm in exactly the same situation.
My hippo always gets a slight case of ich whenever I introduce new animals in the tank (yes I throw away the bag water).
It generally lasts 2-3 days, and it's minimal (approx 8-10 tiny spots). He scratches a little and gets a clean from the 3 shrimps, and he's fine.
This has been going on for about a month and a half now, and I have had roughly three "cycles" of ich. It only affects the tang and no other fish. The UV
has been running all along save a week, when it was leaking and I had to fix it. One slightly major outbreak happened after that.
I would say I'm struggling to defeat ich. The UV
is adequate in size and flow so I'm not planning anything else beside feeding plenty of vitamins, greens and a little garlic when the spots are showing.
My guess is that this kind of setup (UV
+shrimps) prevents the fish from getting a serious case of ich, yet does not totally wipe out the protozoans or whatever they are. I'm in no condition to QT the fish and leave the tank empty so I guess I'll just have to wait and see, fingers crossed.