Our Mbu has pretty much stopped growing at 25"-26" in length. By that, I mean that he has not gotten any bigger in the last year. Fish, of course, grow throughout their lives but just at a greatly reduced rate. I have heard of Mbu getting 30" or even 36" but personally think this is the exception to the rule. Many of the big ones I have seen on the web or in person were like ours and right around the 25" range. based on that, I think we can safely operate on judging tank size for a 25" long fish. The 300DD is pretty much the bare minimum I would consider but it seems to be plenty of room for ours. At least in my experience.