[Myka]

I see a backpack, are you a student?

First of all, do you have any tools available to work with? Circular saw? Drill?

The first one is pine, not oak. It looks to be a really good option. You could paint it white or the wall color to match the house trim or walls and it would look pretty sharp. I'd probably pop the boards off the back of it and replace that with 1/2" plywood. You're right though, $60 is too much.

The second one I wouldn't be comfortable putting a 29-gallon tank on top of it as it is. It appears like it would collapse sideways fairly easily. I would remove the lower backing, and put a single backing of 1/2" plywood covering the whole thing. I'd also check under it to see what's supporting the "floor" inside the cabinet. What's the inside measurement from the floor to the shelf? It looks pretty short.

The reason I suggest plywood is because it's one solid piece, and if you tack it on all the way around with nails or screws every 6-8" it adds a significant amount of reinforcement preventing the cabinet from leaning one way or the other and collapsing.