ok i saw your location is in iowa...im SURE there is an LFS somewhere out there. you may not be able to get to one immediately (especially if this blizzard hits that they are talking about so much), but eventually you will be able to.
now it comes back down to being a responsible fish keeper. as has been previously stated, the 20g tank is definitely not big enough for a 10" lungfish, although it can be done VERY temporarily, and you apparently have no choice at this point. you should only even plan on keeping the lungfish if you are indeed planning on getting a larger tank (and sooner than later).
having said that, when is the soonest you can get to an LFS? virtually all LFS have some sort of bacteria supplement you can add to a tank to help get things established, you will need to do this or cycle the tank with the lungfish in it (which is not a good choice). in the mean time, do frequent partial water changes, maybe 10-25% every day to help keep the ammonia and nitrite down. again, the priority here is to get to an LFS and get something that will help cycle the tank.
at 10", the fish can go a while without food, i would advise to stop feeding the fish until you can get the tank situation taken care of. if you have further questions, let us know--
--solomon
