Although ADF stay fairly small, and breathe air, not water, I think they would be happier in a larger tank. IMO, upgrade to at least a 5g tank, with 10g being an even better option.
Having a filter and heater is good. Try to keep their water temp about 74-76 F. Keep up on your water changes, filter maintenance, and gravel vacuuming, and they should stay healthy.
They'll appreciate some live, or artificial plants, both to use as hides, and also to block some of the tank light when they need a shadowed place to rest. They primarily feed from the tank bottom, so a sand substrate would be preferable to gravel so they don't accidentally ingest a small rock. You can use a small saucer to feed them on. They'll quickly learn to come when the saucer is put into the tank. They'll love frozen blood worms, brine shrimp, etc. You really should only feed them 3-4 times per week.
If you go with a 10g, I would only fill it to within a couple inches of the top to be sure they can reach the water surface to breathe. And, a tank cover is a good idea, as they can sometimes crawl over the tank sides, or crawl up cords and airlines, etc, and escape.
I was given a 10 gallon! I only need to buy a hood and get it set up and cycled!!! 
