new QT water or Re-use?

[George]

Originally Posted by jorjef

This could be short sighted and way off but if you are QT'ing a fish and and shows no signs of illness or if you just treat with something broad spectrum why would you need to change water. Wouldn't you be killing what ever the broad spectrum treatment covers and if the fish did develop something that wasn't covered you would treat for it until it clears...That being said I never have had a QT tank so......

It's more complicated than that. First of all, when people talking about broad spectrum in fish world, they are referring to antibiotics, both gram-positive and gram-negative antibiotics.
Then there are nasty stuff that are not bacterial, like parasites, virus, and such that don't react to antibiotics. You need some other medicines for them.
For some parasites, when they are in cyst stage, no known medicine can kill them (at least no medicine that you and I can get our hands on). And the cysts can survie some extreme condidion like high temp and very low salinity.
It's a good practice to clean your QT tanks and change all water after one round of QT.