Ottos prefer to be in groups and will be more active in a group. I have 1 in a 45ltr tank and I hardly ever see it while the 2 I have in a 70ltr and the 4 in the 125ltr are all very active and playful. I never feed my ottos, there is always enough algae for them to live on - and if not they could always have some of the Corys food - though I don't ever see them going for it. They cost ~£2.00 each. I don't know how long they live.