Just an update. He wouldn't eat from the net and then strainer I sequestered in him in my main tank. I hadn't set up a hospital tank again because I only had a broken filter, but I ordered a new one and it arrived today so he is now in a 10 gallon hospital tank with the filter seeded with some media from my main tank. I dosed the hospital tank with Metroplex and salt and voogle.
Main tank seems healthy now with no ich for a few days. Yay.
We'll see if I can save this guy. Once that's sorted the tank will be used as a QT tank for future fish. I currently have one lonely lemon that survived the disaster in my main tank who I hope I can find buddies for again. Kinda hesitant to buy from the same stock I got him from but it's the only local place with lemons, if they even still have them by the time I'm ready and if they're open. But QT tank is ready for them!
And I now only have 3 rainbow tetras which I'd like to bump up to at least 6 which I'd have to order online.