4 ft 150 or 6 ft? Makes a difference. Stuffed with rock or an open aquascape? Also makes a difference. Hippo will eventually outgrow a 4 ft tank and you'll be back here asking why its behaving strangely. I would not personally put an Achilles in anything less than a 6 ft tank - it's a swimmer. I would also not personally put an Achilles and a Powder Blue together in the same system. They are both on the more aggressive end of the tang spectrum. There's always that guy who says he's doing it, but you are asking for trouble. FWIW, at 3.5 inches, neither are juveniles anymore. Are they getting along in QT?