What thickness concrete base?

What thickness should the base for a concrete pond be? I have excavated my
pond area to about 15ft x 8ft x 3ft deep which I then intend to build up
above the surface another 2.5-3 ft giving a depth of approx 5 ft. As I
intend to fit a bottom drain, the literature says that I should prepare a
6inch bed of concrete. Lay the bottom drain pipework on the surface of this
then encase it in another 6 inches. That's a foot of depth that I have lost
to a concrete base. Is this correct or can I get away with a lesser
thickness base?