wELL, YOUDO HAVE A PROBLEM
sorry to say, but I do not hink a UV is the answer(my opinions about uv aside)
FIRST you MUST find the reason and deal with that
you also hve to cure your fish, UV will NOT cure infected fishes
but may only hel.p prevent the spread, BUT if you lost one fish to ick, I would bet they all have it by now. Fix the problem, don't mask it with false hopes of curing a problem.