I have a 10G QT and run a whisper filter off of my 75G to transfer over when needed. That and a heater cost me something like $20...so why wouldn't you QT?
it's just like riding a motorcycle..it's not a matter of if..it's a matter of when.
if you don't qt fish you run a high risk of passing a pathogen into your main tank..remember all the stories we read here about fish with ich, fins rotting off at the LFS?
ich is a common parasite as are parasitic worms gill flukes etc.
let alone any bacterial infections which can be passed into a healthy tank.
that said I have a 10 G and a 20 g qt tank.
I do quarantine. Before, it was in a 10g, which is now my beta tank. Now a 5g hex is my qt/hospital tank. The size you'd want to use just depends what type of fish and how big it is when you get it. Just consider whether the tank is so small it will stress the fish further, because that would lower the new fish's defenses further ontop of the stress from the move. Put in a plant or two and a cave to make him/her feel more at home.
I haven't ever quaranteend because I don't have all that many fish and most of my fish were born in my tank anyhow. I do plan on getting a new BN soon and I think I might QT him in my 10g for awhile before I move him over becasuse I'd be heartbroken if I lost my angelfish or female betta.
Before, I just put them in directly into the tank that they will be housed in... Now, I've started to quarantine them for 3 weeks minimum (I would like to try to keep them for 4 weeks).
I don't, because I don't have a QT tank yet and all of my fish are from the same store. I keep looking around on CL for a free or nearly free small tank or two, but have not yet seen one.
We don't for 2 reasons - don't have a QT yet, and we buy from a reputable mom and pop lfs who QT the new fish and treat them for 2 weeks before making them available for sale
I qt, had a bad instance with a sick angelfish and a fresh start on my tank. theres a couple of places i'd probably feel ok buying a fish from and not qting, but anything shipped or from central filtration is getting qt.
i dont normally quarantine fish, but im contemplating setting up a qt when i move to my new house.
My main worry with new fish isnt spreading disease (my LFS has great quality fish), but the lack of food available for them. New fish are generally much smaller than the current tank residents, and are too chicken to attempt taking some food.