This was written by Joel some time back and should cover most everything.
I have never found Kick Ich and similar products to work well if at all. I am betting the reason your fish looked better the day after starting treatment is because the parasites on your fish matured and fell off to reproduce. The meds do not treat the parasites living on your fish, they kill their off spring during their free swimming stage.
Get the fish out of the tank and get them in a stable system where you can use an ionic or citrate type copper ASAP. (except for the powder blue tang - more on it later) It is imperative that your water quality stays correct in your quarantine system, test and correct as needed. I would suggest that you buy Sea Chem's Cupramine copper treatment & buy either Red Sea's or Instant Ocean's copper test kit. I suggest these kits because they are correct for measuring cupramine's level. (some test kits are chelated copper and would read incorrectly)In the quarantine tank, Maintain the copper level @ .20 to .25 PPM for no less than 14 days even if the fish look cured. On the first day of treatment add approximately 1 Ml. of Cupramine per 10 gallons of water, 48 hours later test the copper level and adjust accordingly, (example; if your test shows that there is .10 PPM copper, add a half dose "5 Ml. per gallon" ) Test the copper level every few days and adjust accordingly. After 14 days, start performing small water changes and incorporate a good quality activated carbon into the filtration system that is maintaining the quarantine system.The carbon will reduce residue copper in the water.Keep them in the system for another 2 weeks of so and watch them closely. If necessary, repeat the entire process if the parasite starts to show up again. Add Salt water Vita chem to your fish food and to the water of the quarantine system to help boost the fishes immune system too.
As per your powder Blue, that is a tough one. As Tim Indicated, they are not real fond of copper treatments and often die from it. How I deal with Powder Blue tangs (which is not often - I avoid them because of how problematic they are) is a 30 gallon or larger bare bottom quarantine aquarium that is filtered & heated appropriately. Over several days, bring the S.G down to 1.010 and maintain it at his level for about 2-4 weeks. The "Ich" like parasites most likely will not survive the lower salt level. After a month or so, slowly bring the S.G back up to a more normal range (1.023 to 1.025-ish) After that, I don't have good advise, putting it back into the main aquarium could start the entire thing all over for you. If I were going to keep a Powder Blue, it would be the absolute last thing added to the tank to help minimize out brakes caused from the introduction of a new fish or invert. (rocks & corals can carry eggs or cysts too). A very small % of Powder Blue tangs do well, it is not common to have one survive long term or at least with out issues. The rare occasion that one does well with out significant effort on the part of the aquarist is some what of a fluke or just plane old luck.
Fresh water dips can be a good method of ridding the parasites from the fishes body but it will not kill the off spring living in the tank. I often fresh water dip our fish if I suspect that there is a problem but I also simultaneously treat with an appropriate medication for what ever the situation is. DO NOT USE RO/DI WATER to fresh water dip your fish. It is imperative that the fresh water be the same temp & PH as your aquarium water. RO/DI should be neutral (PH of 7.0) Either use dechlorinated tap water if your tap water is similar to your aquarium or mix up some buffer into your ro/di water until you have achieved a similar parameter to your aquarium water.
As per the 125, a few things can be done. Just simply not having fish in the aquarium for 4-6 weeks will stop the active parasite population, they need hosts to keep the life cycle going. The UV sterilizers are not going to do much, they are too small and have many limitations to them. (another long subject) Put the 18 watt on the system that you put the powder blue in, it might be more useful there. Some water changes and substrate vacuuming will decrease the parasite population too. If your a fan of UV sterilizers, a 40 watt or larger with a water flow of 200 GPH or so would do much better. It's large enough that with a slower flow rate the exposure to the light will have a much more effective kill rate. If you want to do Better, get an Ozone Generator and use your protein skimmer as the reactor. This will serve many beneficial functions. Ozone will kill & destroy just about everything it comes in contact with. It is very efficient at killing parasites (UV is very limited), it is very efficient at killing virus and bacterium (UV is not effective on either) and it will significantly improve your water quality. Ozone will greatly decrease the organic waste load in your aquarium decreasing the rate of organic build up (nitrates - phosphates, etc..) It won't get rid of existing values but will slow down production a great deal. You must use a controller with the ozone generator but Red Sea makes a great little all in one set up that is very simple to use. It is called the Aquazone 200 deluxe. If you have the extruded Euro Reef protein skimmer, you will need to replace it with a better one if your going to use ozone. The ozone will brake down the lower cost materials. If it's a cell cast acrylic type as most of the euro reef skimmers are, you have no worries.