You could use acrylic, but it'd have to be very thick to be rigid enough. The problem with flexing is that it tears your silicone seals. Acrylic rigid enough is so thick it's more expensive than glass. You won't need 10mm glass for the weir. 7 or 8 is plenty strong enough given it's not very wide. Some braceless tanks use 10mm glass!
If you want to be sure, there's calculators that take into account the deflection of float glass over it's length and give you a safety factor, and you can work out the thickness of glass you'll need as a minimum. 5mm would be strong enough, but wouldn't take much of a knock from a rockfall, but 7mm is plenty.