i could throw the sofa out and have a 6ft tanknot sure the kids n hubby would appriciate that though...ive got a tank in the kitchen(25lts) was going to be a betta tank but now houses a rescued goldie!!and i have the "main" tank in the living room 75L..well its a 80L but only holds 75L after the deco and gravel....
Any of the 'rules' you hear quotes (e.g. one inch per gallon) general take into account decor, substrate etc so even though you have a 80l tank with 75l water it's probably ok to go with '80l' tank in terms of stocking etc.