You're going to need at least a standard 55, which is 4ftX1ft, to house an adult of these. They can get over a foot long! Also, an Eel isn't helping you're problem. I assume you bought this fish at a large size, because in such a tiny tank it couldn't have grown to that size. I would say a 75 gallon aquarium, measuring 4ftX1.5ft will hold the Eel and Devil provided high filtration and water changes. These are big fish you're working with, not little guppies. If you like these fish, you have to get a bugger tank. No two ways about it. They will either be stunted and live a miserable, painful life, or just downright die. A 55 for the RD alone or a 75+ for the RD and an Eel.
Im not sure on how bug the Eel gets though.
Also, not trying to be rude. Im legitimately trying to help you here.
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