Powders do need a larger tank and better environment.
An established system is where the live rock is full of beneficial fauna, the live sand is loaded with beneficial critters and yes, water parameters are real stable.
You do need to remove the tang in My honest opinion.
You should also NEVER bring fish straight home and out them in a display tank. If indeed the tang does have ICH then every fish in there has been exposed and they will all die if not treated. New arrivals need to beput into QT first to watch them for several WEEKS to ensure you are not introducing any unwanted bacteria, ich etc.