Sorry guys maybe i should of explained a bit better, the windows themselves will be 2x2ft, the pond height is gna be 5ft, all in total, 12ftx5x5 will be the overall pond size with 4 viewing windows at 12mm, thats what i was thinking anyway.
The reason i was thinking 12mm was for the sheer length of it, but as it was said earlier it goes on the height not the length so 10mm may be good enough.
Anyway frames tomorrow ey m8, well this is moving along nicly, alot faster than i thought, great progress m8 and good luck on the fill