Love this!
I LOVE posts like this! Like everyone else, I can't wait to see it finished! If it were me, and I'm not sure you want my opinions, if I was going to paint it I would have gone with MDF or particle board instead of plywood. I've often thought about building a stand for a large tank. I love working with wood, and love projects like this. What I've always wanted to do was build a stand that was painted, but that was trimmed with solid oak. Oak strips on the corners and edges, and around the outside perimeters of the doors.
I'm curious, did you have any trouble with any of the 2x4's being warped? Dimensional lumber isn't the quality it used to be, and I would worry about finding straight wood. I'm hoping the 2x4 you bought is kiln dried, so that it doesn't warp later. Semi-green wood has a nasty tendency to warp and twist as it dries. Also, I love the metal braces, I'm sure they added an incredible amount of rigidity. Not sure I would have used as many, but better to have too many than not enough. I also like that you mitered the top and bottom frames, and used the construction adhesive. Love your attention to the small details!
No matter how it ends up looking on the outside, it certainly is a very strong and rugged stand. Looks like you did a great job! Kudos!
I hope your house has a slab floor... LOL