Well, he knows how BIG it is, he knows how many litres it holds, he just doesn't know how many gallons it holds! Give him a break, I'd have to guestimate myself!
I'm sure he knows how big it is in liters, and the whole world doesn't revolve around us and our gallons, right? He might even have visited a forum that used primarily liters to recommend stocking levels, who knows. I'm not saying he did, I just mean that at times we Americans can be a bit... Americentric??, don't you agree?