Originally Posted by shaggylawson
Since the ideal is to empty the water each time a fish has left the QT and to not have any items like sand or rock, etc. to harbor the disease, then I assume it would be necessary to replace the sponge or filter each time as well correct?
Not at all. In fact, you want to keep the sponge filter (or biowheel) in place because it is cycled and carries your beneficial bacteria that will break down the ammonia. Replacing it would require a new cycle.
You don't need to completely sterilize the Q. If the fish shows no signs of disease and you have no reason to suspect infection or parasites to be present in the Q, then just do a large water change to keep nitrates low and adjust salinity.
When I first add a fish to Q I keep the salinity at 1.016. After 2 weeks I begin to slowly raise the salinity until it reaches 1.024, matching the display. Then I move the fish to the display, usually after 4 weeks. At this point I do a large water change on the Q, lowering the salinity back to 1.016. I have never found this to be harmful to the filtration. FYI, a lower salinity is said to be less stressful for the fish over the short term and harmful to parasites. You just have to watch alkalinity a little closer.
If a fish dies in Q, then I run my UV for a week or 2 without other fish in the Q. My theory is that this should kill most harmful pathogens, not having a host and being exposed to UV.