If you specifically said you wanted open top, and sent him the schematics you posted here, I would politely ask him to rebuild the tank with 1" (you paying the difference between what 1" would cost above and beyond 3/4"). I'm sure he can sell the tank to someone else, and more importantly, he should have run his computer program before the tank was complete, which would have given you the chance to bump up to 1". IMO he is on the hook for this - he is the tank builder, its not the customers responsibility to know what acrylic thickness is needed. You tell him what design you want, and he tells you what is needed to build it. If he screws this up, I think its on him to fix it.