A fry tank, medical tank, and QT tank, a sponge filter is absolutely the best choice. The sponge provides food for fry. Removes excess waste and processes it. Provides oxygen which is needed in all cases. Easy to remove and rinse.
I would suggest, if you can find one, to get a sponge filter that is run by an airstone instead of the standard plug in the tubing big bubble type. You will get a lot more oxygenation and a lot more water movement.
The link shows some really good options for use in a quatantine tank/fry tank. With the multiple filters in one setup, you can remove half of them if you treat with something that is likely going to kill your beneficial bacteria and keep them in tank water to be put back in after the treatment. This keeps bacteria alive for a short time (48 hours or more I think with some circulation) so you can keep the tank going instead of losing all your beneficial bacteria.