Well, I am again battling ick (******) right now myself and I have put my guys in my 30 and 10 gallon hosipital tanks for the next couple of weeks. You got a few choices but judging from your reply you won't like either one I bet!
1. If the ick visibly appears on the fish you could fresh water dip him but...
make sure to buffer the PH at the right level and the temperature so that it matches your (DT) display tank. I would only use this as a last resort as it stresses the fish out and will inadvertently make your tang even more suceptible to the ick once it is returned to your main tank where it is still present.
Fresh water dips will of course do absolutely nothing for any Ick in your tank!
That is where you are going to have to "face" the music like me and "treat" the tank and the fish or get the fish the heck outta there and along with all others and leave the tank empty of all fish for at least three weeks. That is where I am right now myself! It sucks but it has to be done.
2. You could buy a 10-20 gallon hosipital/QT tank. I know you most likely don't want to spend the money or do not have it, etc. etc. but it will cost around $20-40 for the new tank with lights and a hood which is not any more than a new powder brown will costs you if the ick takes over on the one you got. If your tank is a fish only with no corals or inverts. then hyposalinity for a min. of three weeks is in order. PLEASE use a refractometer to ensure that you have the Specific Gravity down to the proper 1.009 and do it over the course of at least 48 hours, I prefer 72 hours myself..but whatever!!!!
Note: The tang's scratching is it's natural way of trying to fight off the parasite. Tangs often can not do this effectively in a small closed system as they have a thinner slime coat than most other marine fish and the parasite can more easily "embed" itself onto your fish's skin. I am sorry to say that if you do start to see the visible signs of ick (******) then it is only a matter of time before your tank is so over ran with the ick parasite that eventually you will start to lose your fish(es) and don't believe your LFS if they say that ick is ALWAYS in the tank. That is not true, but ick is always in their tanks b/c of the larger amounts of fish that they stock through their businesses but that does not mean that it has to be in your display tank too! You can eliminate it completely if you treat what you got right now and start to QT for at least three weeks from now on. That certainly includes liverock, etc. that is added too! That is how I got my ick in my DT. Please be proactive and either get a second smaller tank or hypo your main tank. If you can't afford the "other" tank option, then take out any live rock and inverts and put them in a large "Rubbermaid" container or 5 gallon buckets for three weeks with a cheaper powerhead and a small heater so that you can hypo your main tank.
Either way...goodluck my friend and give those parasites...
well you know!
