Hennie if I change as sizes then I come out at 12mm.
My tank is 1800 x 700 x 700 high, and I used 12mm glass.
Must the btm of the tank thickness increase or will 12mm do?
That would depend on the possibility of movement, maximum deflection of the stand, and on whether the tank bottom is fully supported over the total area, or just on the outsides - if the tank is placed on a thin layer of Styrofoam on a concrete slab which support it over the full bottom area, then 12mm would most likely be OK (I used 12mm for my tank's bottom glass as well). If it's placed on a wooden stand, with one side resting on a brick wall as you originally planned, then you should stick with the recommended bottom thickness as calculated.
Over the past few days Ive been surfing the web and always seem to come across CO2 cylinders is this a necessity or not if so how does it work?
CO2 is used to dissolve crushed coral substrate inside a calcium reactor. If you are planning on keeping many hard corals, and especially SPS corals, you will need to supplement calcium. This can be done with a calcium reactor, of by adding chemicals or dripping lime water (kalkwasser). To start up with, I would not stress about CO2 - rather use additives when needed for the first 6 months or so.
Another question how many times must the water be turned over per hour?
Like so many other things, it depends...
A SPS tank should have as much water movement as possible (20x to 40x are not uncommon). A LPS and/or soft coral tank would be fine with a total turnover rate of 6 to 10 times per hour.
Keep in mind that "turnover" refers to total water movement, NOT flow through the sump. You could have (say) 3x turnover through the sump and (say) 10x turnover created by power heads and/or closed loop pumps.
More important than turnover, though, is turbulence. You want what's called "chaotic" water movement, with eddies and currents constantly changing direction (swirling, like when waves break...), rather than laminar flow with water always moving in one direction (like in a river).
Hennie