Earlier this year I built a stand for a 10gallon foot print out of single 2"x12"x12ft (meant to get a 10ft, but wasn't paying attention, so I have some excess chunks)
Two uprights on the end, with simple circular saw/chisel dados holding two horizontals (top and bottom), with some adjustable feet on the uprights.
Cut the plank in half for transport, brought it home, laid the two halves flat next to each other, and butted the nice end up against something even and sturdy.
Make a cut for the uprights an inch or two longer then where you want the base of your aquarium to be, and leave the pieces still butted up against the wall or whatever. mark out where the top and bottom shelves will be and cut the dados (I did mine just a hair under 1.5" wide, so it would be a snug fit) - I just strapped down two boards on either side to guide the circular saw, and did multiple cuts across both planks, just a bit under 1" deep, and then chisel out the remainder. Do this again for the other shelf.
take the remainder of the wood, but it up against the wall again, and cut the shelves.
cutting across both halves at the same time ensures the cuts and dados will be at the exact same height.
Then check for fit, sand a bit if necessary, add glue, and smack the shelves into the upright, smack the other upright onto the shelves, and add some screws to pull everything together as the glue dries.
Really easy design, fairly cheap, doesn't take much planning/measurement (as long as you are careful with the cutting, and do both pieces simultaneously), and it's plenty strong.
First pic is the stand, second is a closeup of the upper shelf/upright joint.