according to the manufacturer website, Pondtite will take 1.3L to cover 2 square metres PER COAT. having done several large enclosures, i think they are being very overly conservative and maybe using the coloured product, which is substantially thicker
and from memory you should be laying down 3-4 coats for something that will be holding water, and the clear stuff is so economical and goes on so easy that i do 3 -4 coats anyway even just for sealing against 'splashing', just so i have 'room' to use a bit of sandpaper and knock off the shine.