I would recommend getting the Cuprisorb or at least running carbon.
Hard to say how long the fish will be ok there for. Really depends on the size, your filtration, and their feeding needs. Odds are you will need to be very careful since that is such a small tank. Ammonia will not be your friend and could build up quickly.
I'm guessing you have a FOWLR, so the fact that you used copper in your DT isn't a huge issue. If you can get Cuprisorb in there tomorrow that would be great and you should be able to add the fish back in 24 hrs after adding it. If you use activated carbon instead, you may want to give it an extra 24 hrs. The impact of the water clarifier will only last a few days anyway but you need to reduce the copper level to keep it from shocking the fish before putting them back in.. Leave the carbon/cuprisorb run for at least a week to help pull the copper out of the rocks and sand.
If your fish can tolerate it, you may not want to feed them in the 5 gallon aquarium. If you do have to feed them, do it very lightly and clean the tank immediately afterward.