Originally Posted by shrimpgoblin
I recently battled a columnaris outbreak (lost 5 fish within the span of 48 hours). I pulled every remaining fish that seemed to be exhibiting any signs of infection and put them into quarantine then began treatment (Kanaplex + Furan 2). All have made it through the treatment and now show no visible signs of columnaris infection. Some do appear a bit sluggish but none are gasping for air or have any white patches. My main question is when do I return them to my main tank? I don't want to do it too soon and risk the health of my non-infected fish.
As Colin stated, if you did not treat the main tank, you could easily re-infect your fish.
Studies have shown that true Columnaris can survive without a host for as long as 60 days while many sites say only 30-35 days. I trust the studies over the websites.
If the kanaplex +Furan 2 treatment cured your fish, I'd use that same formula in your main tank then wait for at least the 30 days. At that point you will want to use a bacteria booster ( i.e Fritzyme #7 or filter material from an established tank that was not infected) as Kanamycin ( Kanaplex) can kill off nitrifying bacteria. You will need to monitor ammonia levels after reintroducing the fish back to the tank until the bacteria bed stabilizes again.
If the hospital tank you used for treatment is too small, you'll want to consider setting up a "quarantine" tank that will serve has a version of the main tank where the fish can live as they would in the main tank for an unlimited amount of time. It will also serve as a good test tank for future purchases to prevent introducing any pathogens into your main tank again. Quarantine times should also be at least 30 days as there are some parasites with life cycles that long.
Hope this helps.