Generally what you want to do is run a small filter in your main tank so it gets the beneficial bacteria, a small sponge filter works fine. Then when you have to QT a sick fish, just take your sponge and put in the QT. From that point on the fish will supply the ammo and you'll basically have an instant cycled tank.
However, in a hospital tank, you should be doing so many frequent PWC
's, since providing clean water is a big step in treating sick fish, that there may not be that much time for ammo to build up to keep BB alive. In all honesty, instead of running a filter, you could literally just run an air-stone to keep oxygen circulating. Depending on the illness, you could treat from 1week (early fungal infection) to 1 month for your more severe illnesses).
The best time to use a backup filter is when you QT new stock for about
1 month (LFS
fish) to 2 months (wild caught fish). In that case, you would regular PWC
's. Again, the fish provide the ammo, but this time enough should build up to keep your stock happy.
75g Planted Barbs Denison, BlackRuby, Tiger, Albino Tiger, Green Tiger, Cherry, Gold, Checkered, Odessa.
NonBarbs Cardinal, Neon, RummyNose, Bloodfin Tetras, Z. Danios. 1RTS. Amano, Bamboo, Ghost & RCS. 20g Long Shrimp Tank RCS, Tiger, Amano.