Float glass is normal glass that most average tanks are made out of. Opti-white is a higher clarity. It's actually the same glass used to make eye glasses hence why it's more expensive. It's actually something to do with the the ammount of iron in the glass I believe
Check this picture out Thomas. It's not brilliant but gives you an idea:
If you look at the two glass squares that look like coasters, one is greener than the other. The greener one is float glass, the clearer one is opti-white. You really do see the difference in real life. I was gob-smacked!
I'm tempted by the AE tanks if Richard can get them in. I'm also tempted by the little 30cm cubes that Dan and Graeme had setup for the show. Apparently they are only Â£80, which is far cheaper than I thought they'd be!