I have a 1930's house with a Downing Toilet. Toilet is in good shape, but uses up a lot of water per flush. I would like to replace but the supply line is 18 inches off the floor. Modern toilets have a seat hight of 15 inches which makes the tank at 15 inches. Making the tank in the way of the supply line. I do not want to make this a big effort then needs to be. Is there a toilet that exist out there that is more efficient and has a tank higher then 15 inches?