I worry that even if you QT them it won't necessarily prevent them from getting ill again, in part because I think they are getting stressed. Adding the rocks and improving water quality is a huge step in the right direction. But (as much as no one wants to hear this) the hippo and sailfin really need a 6ft long tank (180g min) to live stress-free. When they are stressed, they get sick, and I suspect that even if you treat them for ich, they will still be stressed and may come down with something else or die prematurely from stress. The butterflys are both peaceful fish and the activity level of the tangs may be stressing them out, causing them to come down with ich. I can't say for the clowns, they are usually fairly hardy and I suspect if you can keep them eating well then they will recover on their own. My personal recommendation to you would be to return the hippo and sailfin to your LFS for store credit, and once everyone else heals up and is doing well, then consider adding a smaller tang that would do okay in a 125g (yellow tang, purple tang, convict tang, or any of the Ctenochaetus family). There are a ton of great tangs that you can add to a 125g that can live there long term! There are also a lot of tangs that are super prone to getting ich - the powder blue and powder brown tangs come to mind, so I'd stay away from those (trust me, I'd love to have a powder blue in my tank, but seems that all of my "favorite fish" are the ones that simply won't survive in my tank, so I have to admire them from afar instead). I also want to point out that you still have a lot of room to add more fish in your tank over time, and ideally you want to add the more peaceful fish before the tangs to minimize stress (tangs and dwarf angels should be close to your final additions). If you have other fish you want to add, then I'd consider adding them first before you add a tang or two back into your tank. Butterflies are notoriously hard to keep (as you and countless others discovered with the copperband), so I really can't say whether your two butterflies are going to recover or not, and even moving them to QT could stress them out too much. The best way to prevent ich is to keep your fish minimally stressed and eating well. I really hope everyone pulls through for you, even if you do go the QT route!