Mine's really active tho. Always swimming around even when the lights are on. But that's maybe bacause mine's still young. Perhaps as they mature, they get lazier and lazier. But i think a standard 50 is good enough for a single gulper. Im not planning on adding another one in there. For 2 above, maybe a 75 or a 120 gallon would do.
On a side note however.....i have a contact here in the philippines, who just got another shipment of little gulpers and can sell them to me cheaper. Veeerrrryyyy tempting to start a gulper community

problem is i dont have space left for another big tank haha