Steel support beams in the walls would have worked fine. Unfortunately, I did not have access to enough steel for the legs too. The greenboard is now up in the fishroom. Mud and tape tomorrow as well as finish the plywood for the stand. I did add two 2x4 headers above the six existing 2x4s which support each end of the I-beam.
Since my sump and a lot of the equipment will sit beneath the stand, I wanted all the access I could get....ie...no center supports.
I am going to use "car wash" board on the room's walls and ceiling. Its comes in 8'x4' white textured formica-like sheets and will be attached to the greenboard using small rivets. All seems will be calked to give an air tight seal. Our public aquarium has used it with great success in its wet areas. Works great with saltwater and humidity.