its always been a tough ? about how long aros can stay in smaller tanks. Really its all about how you fish looks as he swims. Any sign of trouble turning then its time to relocate. I've seen 21 inch aros turn in a 75 gallon. Long story why thaty fish was in there. But then theres ppl that keep aros in 18 inch wide tanks for life. Is this ok? Another touchy subject but for me i have a 4 inch jar in a 55 gallon with a clown knife, palmas bichir, a gold sev, and a stone fish. Now this wont do for life but it could last for up to a year as long as fish arent overfeed. Just watch your fish and his movements will tell you if he needs a new home