I would recommend 2x4s and plywood also. Be sure to cover the wood with some type of sealant. I've used stain and polyurethane before, and I've used Rustoleum's gloss black with no issues.
One thing you may want to do is leave room to expand. Fishkeepers have a tendency to want the next big thing in a couple months. Maybe make it so you can swap out the 3 10's with 3 20H's. Just be sure to leave enough room to do maintenance.
You could probably get away with 1/2" plywood too but I would go with 3/4" just to be safe and keep everything sturdy.