if probably go up to 40, sounds like thats what hes after. hes already done the hard work getting it established in captivity, assuming it was wild caught, so as long as you introduce him properly to your tank, you shouldnt have any problems. could use a QT for a bit just to make sure hes feeding, ive never used one though. depends how much you want it i suppose, they are nice my cousin has a decent sized one, its stunning. any more than 40 and id start looking at buying my own juvi one for cheaper, if they're available. factoring in postage though if needed, might still work out more expensive unless you stumble upon one locally.