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I have a female EBJD (Electric blue jack dempsey) in my 40g breeder. Jetajockey keeps a pair of EBJD's in a 40g as well. She's about 7" and does fine. Water conditions stay under acceptable limits.
She is a pretty relaxed fish from what I have seen so far with her, she doesn't just swim around a lot. But she always comes out and up to the glass when I come up to it, and she is also very friendly to her two tankmates. (Ropefish bichir and senegal bichir)
Remember that each fish has it's own personality. If you get something and it just looks really cramped in the tank and doesn't seem happy, re-home it and try something else.