I would recommend against this, and for you to rehome the Zig Zag eel pronto. That can get to 20" + depending upon what it actually is (Zig zag often gets used for several species of eel).I would recomned a Zig Zag Eel They are really cool I have one in my tank and he is verry personable. I also have a Raphiel Catfish I love but he only comes out at night. Glo fish are neet fish also...... I like to go to a non chain store to look for my fish you get a much broader selection at them.... Good luck though I wish the best for you!
If you kept either fish in a small 10 gallon for any length of time it's possible you already deformed them due to stunting. Take this as a hard lesson to research before you buy and not listen to the often mindless advice of people looking to just make a buck.
As for the topic, I think the best bet is to use RO water but to do as was suggested and re-mineralize it. Cherries and snails are both good critters to keep with this size and can do well enough when you have to leave the tank be for a bit. If the temp of the tank is a concern, buy a high quality heater.