Hello,
I went through some similar problems when I up graded to M.H. but I already had clowns and three tangs in there. The Tangs did not get ich but what I did get was algae (greenish, hairy) never had I had algae grow, so I figured it had to be the M.H. lights, the temp. was reading 79-81 at times in the evening after arriving from work, I tried water from another company and that helped the problem. I always use only R/O water from glacier or culligan, but the most noticeable change was due to removing the top and letting the air flow throught the top of the water surface, my temp lowered to @75-77 in the evenings and my algae slowly but surely started disappearing, (5 turbo snails also aided in the problems) this was @ a year ago and since then my temp has been steady at 75-77 and I do not have algae and my M.H. lights are on for @ 9 hours. the only big change was the temp and I figured that this was the cause of my problems cuz ever since then it algae has not shown up and my corals also seem to respond better, as for my tangs (yellow, blue, blonde naso) they never showed any signs of ich.
p.s. My friend uses tap water and even after using Prime additive, he still gets algae once in while, but our tap water is really bad.
hope this helps