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At least a 50g for one. They will get to be super aggressive and huge. Before I rehomed mine, she was 10" and a complete pita. She would take pellets from my fingers but during WC's and tank cleanings would literally draw blood. What you have is a recipe for disaster going on. Even a mated pair in a 75g is difficult as one (usually the female) could easily turn on the other. Good luck !!!!