you'll hear 180, 240 and 300s will be good for life.. ..but realistically you'll be more likely to need a 9x4x3 to keep a silver for life.. they grow huge, fast and even if you get lucky and buy an aro that by genes actually won't grow any bigger than 30" you'd still want to be prepared for a 35+ aro.. a 180 will buy you some time but I'd go with a 8x30x30.. the 180 will be a bit on the smaller side when the normal aros' growth starts slowing down, that extra width could give you an extra couple of years.. jmo.