while they can get to 30"+, you'd be hard-pressed to find one reaching 24" in aquarium...its gonna take a looooong time if at all. in fact reaching 20" is no easy task. it seems growth slows quite a bit after they hit 16"-18". still they can reach 16" or so relatively quickly, in couple years perhaps if you start with a juvenile. so yes you should upgrade the tank first before getting the endli. i would suggest something at least 30" wide in order to keep it for life, of course wider the better