There are tons of castors out there that have higher weight limits than these, you have to go beyond Rona and Canadian Tire tho to get them, and they arn't cheap. After doing a few quick searches on the net they would go for about $175-$200 a castor.
The stand looks good and strong, the wheels will hold the load, but others have the point on how are you going to do the final level of the tank. Seeing that you are going to block it up now make sure the blocks are close to the wheels as that where the support is needed, but I am sure that your hubby will look after that as it is a very sound design.