quarantine advice

I have had some really bad quarantine results. I have a tank in place - 350
gals, I think. It has some unwanted gf in it, which I think will serve well
for 'canary' fish. I know I need to run some of the water out and replace
it before I put in this new baby. I plan to do frequent water changes,
rather than setting up a filter. These gf have lived out the winter in this
setup without it, and I have not had good results with the filtration of
this tank in the past. The total fish load will be about 18" in the 350
gals. I think two to three weeks should be enough. Any disagreements?
My past bad experiences have led me to skip the quarantine entirely, but my
last resulting disaster cured that carelessness forever. Mostly, it makes
me think hard about acquiring new fish.