at the bottom see you just used screws to support those tanks what kind are they?
You'll need to use screws appropriate for the weight it's got to hold. Typically, I'd have added vertical wood supports (see pic below), but these are small tanks and I got lazy. I've gotten by with typical wood screws for racks with 10's and even a 30H, but know the strength ratings for the screws you intend to use, or just use the vertical supports. For instance, the tanks in the pic above are about 80# each filled. I used 8 screws total for the shelf, so each has to hold 10#. Just about anything can handle that, so I used deck screws. If you're building a shelf to hold a single 20, that's about 160#. If you use eight screws, each needs to support 20#. Common wood screws will handle that as long as you don't go real small. Someone will probably come along and tell you it's all going to collapse and fall down on you. If you've checked the strength ratings, you can smile and nod, and know otherwise.
Sorry for going off topic. Anyone got more on storage containers?