Well it is up to you ... With a 55 gallon what are you stocking it with? You may not need a QT tank for long. Besides. You can always keep a backup filter running on your 55 gallon and then plop it onto a bucket or back up tank or rubbermaid tub easily for quick QT tank purposes.
You don't have to keep water in the quarantine tank. What I do is place a small bag of filter media in my sump. When new fish arrive, I just fill the Q.tank and dump that small bag in. And I add a bubbler to agitate the water surface too. Once the fish have been quarantined, I transfer the fish to my main tank, rinse the small bag of media in boiling water to kill everything off and place back in my sump to let the bacteria repopulate for the next fish.
im going to be stocking 55 with a convict(maybe a female as well),clown pleco,school of ghostcats,shool of skirt tetra,one or two chinese algae eaters.
i dont know what a sump is but im assuming its part of a canister filter? i do not have one. right now im using a penguin biowheel 350 and a 15gl filter on the otherside til friday.ill be getting an aquaclear 350 to replace that and do the rubbermaid thing and use the 15gl filter as the backup for instant qt bucket...yay tarantula!!!
I started using a quarantine tank for all new fish about a year ago. I think it is a good idea and offers a protection to your established tanks and current live stock.
You do not need to keep it running at all times. I just move an established filter, water, and some substrate/rocks to the quarantine tank.