I wouldnt treat the tank with it unless you want to kill most everything. But if its fishless and live rock less/ and coraless dose away. Thats the purpose for doing it in a separate container.
Just think, h202 adds oxygen and parasites attack the gills too. H202 is a double whammy killing the parasite and adding needed o2
H2O2 does not add usable oxygen to the water by itself. It generally breaks down to water or reacts with other molecules in the environment. The only time you really get molecular oxygen is when animals that have the catalyase enzyme (the reason it bubbles when you use it on a wound) converts it to water and molecular oxygen (bubbles) if its not bubbling it not producing oxygen and not all of it dissolves into the water when it does bubble. H2O2 works in that oxygen atoms are electron hogs, when you bond 2 of them together into a peroxide they fight over the shared electrons and generally break apart to form oxygen radicals (each one takes 1 of the 2 electrons) which are very reactive because Oxygen wants both electrons and so it will find one to steal from another molecule. This is what antioxidants do, give up an electron to radicals. This is also why it is a good antiseptic. Our physiology is adapted to deal with radicals like this with the enzyme catalyase for example, which most microbes and plants don't have. I use a 25% tank water solution of pharmacy 3% H2O2 and have treated coral frags that are overgrowing with algae, zooanthids, LPS, and SPS seems to handle it pretty good (don't really like softies so no experience there), they are POed and close up for a day or so, but all of mine have opened fine and the algae dies off within a day or 2. so much easier than trying to manually remove it.
I like the idea of using something like H2O2 or O3, but really need to see studies showing efficacy on infected fish, resting stages of tomonts etc. If this was so effective people who use ozone should also see the same results i.e disease free tanks. I would be most interested to hear from anyone who uses ozone if they have ever had an outbreak of velvet or crypto and just throw animals into the display. I would love to see some peer reviewed studies showing susceptibility of the parasite and the fish to such treatment as it could have promise, but I take the example of it worked for this one guy with a grain of salt. If it's not reproducible in a controlled study then it could just be luck that it worked.
You can treat the tang and put him back in the tank but he will just get reinfected. Most people do one of two things. 1. practice "ich management" which means just dealing with it as it comes up by feeding heavy and adding live food items or vitamins to the food. OR 2. Qt all fish, run fallow and QT all future fish
I dont see much a benefit to treating only one or two fish every time they show signs of ich. It wont help much in the long run. The ich will still be in the tank and they will get it again as soon as they go back into the DT.
+1
catching your fish is very stressful to then. If you gonna get rid of the ich catch and treat once go fallow and then QT to give the best chance of avoiding the problem in the future. It took me 3 days to trap out all my fish when ich came knocking. I ended up loosing 2 tangs and a lawnmower blenny to it. My case of ich came in as a tomont on a coral. From now on all items going into my tank will get a through QT/Treatment as I have plans for some expensive fish later and don't want to watch the animal suffer and my money go into the toilet.
End of the day pick a strategy and understand the risks of each. ich management means any stress in the tank and you can get an outbreak and some sensitive species may never work in your tank, or you could bring in tank killers like velvet if you choose not to QT. QT means you might lose a fish in the QT process if it does not tolerate it well, but you only lose the single fish and you are ready to medicate or treat if you see disease and don't have to try to get everyone out of the tank, which also costs you valuable teatment time, or destroy your rock work/corals chasing fish around the tank. QT tanks dont' have to be barren if you choose the watch em' method you could have a nice little tank with live rock and microfauna, just smaller so if you have to treat you can catch them easier.
It's really all about risk management what you are willing to tolerate and accept.