Why not start up an brackish water tank and throw in a couple archer fish. I know the tank is too small for the eel but at least then its in the right kind of water. I don't see you getting rid of the archer fish anytime soon so at least give it as good a home as you can.
Sell the fish which must be sold and keep something smaller. And even if it sat in freshwater at the LFS, someone who knows how to take care of it might buy it. At the LFS it has a chance. In your tank it is definitely dead.