OK, having used quarantine tanks for my freshwater fish since I started will offer some suggestions.
Ideally you would want to quarantine fish of the same type together. ie say you bought 2 gourami and a tetra, keep the 2 gourami together and quarantine the tetra separate.
At minimum quarantine for 3-4 weeks...some fish carry diseases which will not show up until around 2 weeks or so.
When setting up a quarantine tank, purchase (of course) the tank, heater (one which you can adjust the temperature on is ideal), adjustable will allow you to raise the temperature should you get an ich outbreak, compared to not being able to raise it with a preset thermometer.
NOW..sponge filters are these
, place the filter in your main tank for 4-6 weeks prior to adding it to your quarantine tank, this will allow it to get beneficial bacteria on it. Once you have it seeded, then you can use it in your quarantine tank. If quarantine goes well and no medication is needed for the fish, then simply place the filter back into your main tank, hidden in a corner, doesnt have to run, just so it stays seeded. If medications are used, then the process will have to be started again after washing the filter to kill off any traces of diseases...tap water and a dilute bleach solution.
If medications have been added then the filter will need to be seeded again from the established display tank. Once seeded again, then with the filter staying in the main tank, it can be easily removed and used right away in the quarantine tank if you purchase new fish.
I leave my quarantine tank bare bottom with no substrate, remember this is a quarantine tank, not a display tank, so looks dont really matter. A couple of decorations to allow the fish to seek shelter if needed are ok, but again, if medications are used, then they will need to be cleaned before being used again.
Anytime medications are used, once the fish is cleared of disease and ready for the main tank, I do a complete breakdown of the quarantine tank including a bleach cleaning. It can be setup and ready to go again in less than 24 hours assuming there were problems with disease and medication. I wash everything to be on the safe side EXCEPT the filter, if your fish were disease free then the filter can be stored in the main tank until the quarantine tank is clean and ready to go again.
In summary, adding fish should be done slowly anyway and as such DO NOT rush purchase a fish on the pretense it might be disease free...Quarantine should always be used no matter what you are adding in my opinion.
Hope this helps.