You have a huge tank---but if you got a pump with an inch hose, you could drain into barrels fast enough that your corals would take no harm of the exposure to air [except sponges]...
Fish gather in the lowest part of the sand, where they can be netted.
Water replaced as fast as it went out. Test water for ph change, etc, but it's the fastest, gentlest, least reef-destructive method I know of. Once I learned how to do it, I never had any doubt I could get just about anything out of the tank that I determined had to come out.
I also bought some of the product you mention, but thought better of using it. And the fish recovered on its own with good water and garlic, anyway. Probably I have ich in the tank, but it's been over 2 months since any showed up, so either everyone has it and is keeping it secret, or it's gone. I have mostly gobies, blennies, dragonets and dartfish, with one damsel, and all of them are ich-resistent---so I really don't look to see it turn up again. [What brought it into my tank was a rabbit, and he went back to the lfs, who cured him with garlic and hypo, before he went to another home.]
I wish you luck with yours!