50% + 50% = 75% water change (unless you managed to only suck out the old water molecules )
Hope your angels will get better soon.
Don't think temp is the problem. Afaik Angels are very happy around 26°. I wouldn't worry about trying to get bio filtration on your hospital tank. Sounds like it is nice and big and with the wc's you are doing I wouldn't imagine amonia becoming a problem.
I think you are doing all the right things, now it's just finding the right cure. (I think starting with wunder tonic was a good idea, after that I would perhaps try Furan 2 if I was still unable to diagnose it more accurately)