[Dlaw]

Hobbyist, I think you might be confused as to how RO water works with relation to PH. RO water is as close to pure as we can make it, with no minerals or (hopefully) organics left over. It doesn't matter what the PH is from the system, as there's no buffer, so it will quickly fall. That falling PH will cause more issues than it will solve.

Also, recommending that someone go out and buy a $100+ tank just for quarantine isn't realistic, and neither is doing all of that just to treat both tanks anyway.

OP, what are the water parameters in the tank? PH, nitrate and ammonia levels are the big ones here. You said there's other fish in the tank? What are they, and how many? Have you confirmed they're all alive? Fungal growths are usually caused by a nutrient spike and the bacterial load it usually represents, and usually those are caused by an unnoticed fish death. I would verify no bodies are present, and then proceed with the tetracycline as directed on the packaging.

 

[Dlaw]

He's using straight RO no DI, If he is going that route he needs to add Di to his RO buddy.

I'm confused as to what you mean here Susan? I don't think the OP is using RO at all, or at least it doesn't look like it from the post. If you're referring to the PH/RO relationship, I don't think it matters if it's been deionized or not, does it? The buffers are still removed, regardless of ions present, right?