A tank that big would weigh in excess of one ton, or 2000 pounds. Therefore you would need an extremely sturdy stand. Therefore you will need to use 4x4's because 2x4 and 2x6 couldn't support the weight. They would be ok to put on too of the 4x4 to help distribute the weight, but your main wright bearing supports MUST be 4x4s. Other than that, outside boards are decoration more than anything else. I would suggest about 4 to 6 three foot 4x4s in the front and back, an a few on the sides. Also make sure your placement in the house will be able to bear the weight