So let’s take a step back. You have to choose the route you want to take. There are two ways to treat ich. 1) management and 2) prevention.
Management goes under the theory that we all have ich in our aquarium it only becomes an issue if things are stressed in some way, weakening their immune system and leading to an outbreak. A lot of people will buy UV sterilizers to help manage it as a property sized one will kill the free swimming ich (research the ich life cycle to learn more) but those are expensive for a properly sized one. And cheap ones just likely aren’t killing it.
Prevention or elimination would mean you need to transfer the fish from the display into another tank, treat them and then keep them out of the tank for 76 days. This will starve the ich and kill it outright BUT it can then be reintroduced to the system when you buy new coral or fish if you don’t QT them prior to putting in your tank.