They grow slower because they are stunted, stunting fish shortens their life a lot.They are supposed to be fast growers but my brother had one in a 40gal that actually broke and it ended up (and is still in) a 10 gal tank in our grandparents house lol they seem to grow much slower in a small environment... but a 55 will provide him with some decent growing room so I would say a couple months on him?