Well, a true qt
would be a system separate from the the stores system, with its own water. As long as no other livestock made its way into the tank, you could do that.
I would highly suggest qt
either way. You don't want to introduce anything to your main, it is difficult to get out once its there.
Also you never want to treat the main with any med. Even substrate can absorb them rendering the treatment useless and killing off your biological filters.
The few bucks invested, will save you many bucks (and headaches) for the future.