If you've had ich in your tank and you still have fish in there, then your tank will likely always have ich. Many fish can live (apparently) healthy lives coexisting with ich, that is, they do not seem to be infected by it; however, in all likelihood the parasite is still living in your tank.
The only two ways that have been reliabily shown to be effective at eliminating ich in tanks (and yes, it is possible) are copper and hypo-salinity, neither of which are reef-safe.
The basic "recommended" process would be to remove all the fish from your tank and QT them for at least 6 weeks treating with either copper or hypo-salinity. The ich in your main tank will die without hosts to continue its life-cycle (which is roughly in the six week timeline). If your QT treatment was successful and the ich has been killed, at that point you can safely transfer your fish back into your display tank and it will be ich free (until you either add another fish or water from another tank).
So, either live with ich (I can't follow that process for my tank) and hope for the best or, if you can manage it, use one of the above treatments and really take care of it. If you do the latter, you just make sure you always QT every new fish you put in and you should also treat everything else you put in too (corals, etc) and in particular, make sure no water from another tank gets into your system (ich does live in the watercolumn for some span of its lifecycle).
http://www.petsforum.com/personal/trevor-jones/marineich.html