Sorry for the loss and agree it looked bacterial...probably from wounds sustained by the pair picking on it. Next time, immediately remove to qt, add salt, lower temp, and do lots of water changes. The reasoning is that is that salt is a mild antiseptic and helps with slime production, bacteria multiply rapidly...especially in warmer waters so run the qt tank below 80 and do lots of water changes to keep the bacterial count down.
Then, try to figure out what exactly the problem is and choose the right medication. Also, I’m not convinced Melafix really does anything but YMMV.
Hope the pair have lots of fry for you