No need to go THAT heavy duty-I have a 55gal aquarium on a stand made from 2x2 and 1x4, after all, what does a gallon of water weigh. If you design your stand properly, it doesn't use very much wood. I would use six legs, with a 1x4 top band,another band about 6"-8" above floor with a shelf on it. I really dont see you filling it with water, most likely, you'll have trays sitting in it.

Rick