Better be real careful with the soap!if you only go 15ft high, you won't have enough pressure to knock an ant off a greased banana peel.
2.31 ft = 1 psi. So you set the tank at 15 ft. That shower head is gonna be off the ground 6ft or so, that leaves 9 ft of fall. 9/2.31 = 3.91 psi, not accounting for line friction loss. Most homes have 40 psi or more, so you are talking less than 10% of the normal pressure you have in a home.