yeah...u might wanna consider 12mm since yr tank is a tall one....water pressure increases with height
recommended return flowrate depends on yr total tank vol...min 6times total vol per hour.
max can be anithing
the hole i haf in my tank is enough to fit a couple for 16/22mm pipes
these pipes can carry more than 4500l/hr...if u haf tt kinda flowrate or less...u should be aiming for this