Oh an easy one! *grin*
QT tanks serve 2 purposes: quarantining new fish, and isolating sick fish. In both cases, there will some basic things needed: filter, aeration (if its not a
HOB filter; the
HOB filters usually cause enough surface agitation, which is what gets oxygen into the tank - those bubbles? Don't add any oxygen. They agitate the surface which improves
O2 exchange), a heater. Thats pretty much it. I don't use any gravel or much decor (I will add a ceramic tube or a fake plastic plant if I'm QTing fish that need a hiding place). Its not there permanant home, so only the basics are needed. Plus, if there is disease, all those things are really easy to sterilise.
One actually doesn't need to cycle a QT tank. Most of use keep an extra bit of filter media in our main tanks; if we need to run the QT tank, we put that extra bit in the QT filter; instant cycle! This way theres no need to keep an extra empty tank set up all the time. Do keep in mind, if you need to medicate, the meds will likely kill off the good bacteria as well as the nasty ones, and the tank will cycle all over again. Is another reason having test kits are so important, and why its better to treat in a QT as opposed to killing off the nitrifying bacteria in the main tank.
Lastly we ALL make mistakes. When I started this obsession...erm...hobby *grin* I had 2 angelfish in an 8g tank and cycled the tank with them. THEN I added a common pleco. Didn't realise what I was doing wrong and had no idea the cycle even existed until the plec grew 4 inches in 3 1/2 months and I finally went and did some research. You're in MUCH better shape then I was
LOL