Well that is a hard question to answer. Because not very many people have ever been able to keep them. And of the people that hunted long enough and tried hard enough to get a group, very few kept them for more than a couple years. And then on top of that there are around 16 different species of aces and they get to different lengths. So With the info that we know about them the large ones get to around 16-20'' at fully grown in aquaria. And that is only a speculated size. Meaning no real proof just someone saying I know where a 18''er is, or my buddy has a 20''er. But there is a few confirmed 13'' specimens out there on other forums. And those are all the yellow tail species. The longest red tail that I have seen measured pics of is 11''. And the other species are so rare that there is not much real info on them. And like the ones I have hear, some people are saying they will turn into the more common red and yellow tailed species. And others are saying that they are one of the more rare species. It is just going to be a learning curve for me and anyone that reads my threads on these fish. They are not going anywhere now that i have a group. I have had many thru out my life but never a group. And school is a life essiential for these fish. Trying to keep a single fish is a death sentence for them. They use the school for every day life like eating and protection. ------------Sorry for the long post but it is a hard question to answer truthfully. And if you have any more questions just post them up and I will give them my best and honest shot.