You could add a sponge filter which is just an aerator and the filter itself. Maybe add an elbow of PVC pipe or something so the fish have somewhere to hide.
stephen, when you say treatment, what treatment? If your fish have ich, you should NOT be treating them in your display tank because the medicine (copper, most likely) will kill things in the tank like inverts. There really is no effective treatment that is safe to put in a reef tank--thus, the fish must come out.
That said, you can give your fish a few days to see if their immune systems can overcome the disease (people recommend adding garlic and additives/vitamins--selcon, I think) to the food as immune system boosters. Fish can often overcome disease--as long as they are eating, AND the ich isn't progressing, you can wait and see. If they stop eating and/or it gets worse, it's probably time to act.
the beauty of the nano tank is that unlike big tank it's much easier to collect the fish....you can just put a enough water in a bucket and pull out the rock, extract the fish, then rocks/.corals back in.
ALSO---if you have time--if your fish aren't too sick yet---you need to get water from the nano into the hospital tank. Maybe pull 20% of water per day for a few days and put it in hospital, keep refilling the nano. You don't want to shock your fish into new water---try to get a bare minimum of 5 gallons from the nano into the 10g hospital before you put fish in---otherwise the stress of the move can make it worse.
--fwiw---I have had one tang with popeye and one clown with ich---they both managed to get better in the displays without extraction.