Bige I think your figures are slightly off as a 260 litre tank is a 68 gal, but that is neither nor there. Yes I have the tang still and seems to be settled in and happy, when it becomes too big I will either give it back or will have upgraded by then.
With the Koran, they can reach 'upto' 15inches, that is when they are fully adult, it is then that a minimum 100 gal tank is highly recommended, the one we have is tiny. I am not going to put the fishes health at risk and will like I said either sell them back to my LFS or I will have upgraded to a 6ft+ which ever comes first.