I don't but I should, I have seriously been thinking about doing so. With that said, all the years I have been keeping fish I've only had one incident of disease and that was ich.
Before buying I always look at the fish good for any signs of illness and also look at the others tanks for signs, since all tanks are connected to the same water system. The store I buy from I trust and they will not sale a fish that they know may be sick.
i do now, basically because nearly all my fish are wild caught and sent to the lfs and i pick them up as soon as they come in, and i had one nasty experience were i lost alot of fish, so i learned a hard lesson, i generally keep them seperate for a couple of weeks to observe them and if all seems well then introduce them into my tank slowly.
Most of the diseases that can affect fish are probably in the tank already. As long as the tank is healthy, the fish will be strong enough to stay healthy too. A Q tank won't be nearly as healthy as my show tank, so it's likely to cause more of a problem than it solves. However, I don't keep expensive fish, so in the event of a few deaths before I get the meds out, it's not a big loss anyway.
If I get my fish from fish club/plant club friends, I don't quarantine them -- like my recent Bristlenose pleco (who is such a cute little bugger!). I also didn't technically quarantine my new Killifish, but that's because they went into their own tank. Otherwise I quarantine for at least 6 to 8 weeks.
I always quarantine fish for at least 4 weeks. I had one episode of losing some prized aspidoras to forever instill this as a requirement. I now keep a ten gallon with a small heater and sponge filter going all the time. I leave a rosy barb in there all the time to keep the filter working.
I don't believe in it--they have to acclimate and go through the stress of the quarantine tank...and then go through the entire process again in the disply tank when you deem they're ready.
Drop them in the main tank and give them more of a fighting chance--if they're sick, they're sick...they'll get through it. If not, then they weren't 'fit' for your tank. But I tend to view the aquarium system as a microcosm of an environment: something things will make it, some won't...that's just the nature of it. If one thing dies, it may mean the launch of a beneficial bacterial population superceding the place of another, etc. Things are in flux and healthy as various things fight for their positions and places within the habitat, including illnesses. I just let things take their course.
I quarantine for 24 hours or overnight. Greater time in bare glass tanks causes stress IMO. Stress kills as effectively as any disease. Careful observation is important and that starts at the supplier. I do not buy fish from tanks that have problems. I buy from shops I frequent.
I conduct a 3 stage level 5 haz mat quarantine on all items intended for tank entry. This, of course, includes plants, rocks, and water. The entire process takes 7 months. I use the Gasadro Protocol, including the "optional" hermetic sealing sub routines, but we have all heard too much debate about those, so I won't go into that argument here.
Step 1: Utilizing full containment sterile field principals, I isolate Q-material in the clean room, monitoring appearance, parameters, and bio-metiric data including lab analysis of stool/waste samples every three hours. Length: 1 Month
Step 2: Move Q-material to Close Monitor cell for 5 months, daily checks of appearance and parameters.
Step 3: Conduct final inspection period. For live Q-Material, conduct full dissection and chemical analysis of organ slices.
I made all this up, hehe. I just dump them in the tank after temperature equilibrium.
After introducing several new guppies to my 29 gallon I lost the entire tank, save 2 fish, to acute columnaris disease. On 2 other occasions, I had new gups introduce those nasty camalanus worms into my tank and had a lot of difficulty ridding the tank of them. I learned my lesson and I quarantine everything for a minimum of 3 weeks before introducing them into my main tank. No problems with disease since then. I do take good care of my quarantine tank though, it has 3 resident "feeder" type guppies to keep it going and a little corner filter.
PS - I have some very cool microscopic pictures of those nasty camalanus worms that came out of some of my infected fish if anyone is interested in seeing them. :axe: