danios aren't going to cycle a marine QT tank.
I'd buy a small hang on filter and run it on your sump. When you need it for QT, pull it off the sump and put it on a bare bottom QT with PVC
for hiding spots. You won't want to use substrate or live rock because you'll nuke them anyway if you have to medicate anything. When you're done with the QT, you can tear it down. Just toss the media from the hang on filter, clean it, and replace with new media before you put it back on the sump. That's what I would do anyway.