Shims can be snapped to size, You need a box cutter with a sharp blade, You make an incision on the shim directly below the end to leg. The you can snap of the protruding part. I also use wooden shims in multiples. I usually insert them at the corners. That way they create a space towards the unshimmed end. What this does is to create a place I can insert more shims if I want better support (rarely) or if I need to raise things more.
If I add new shims in that space, it effectively raises the corner and that means I can add a shim there. Once I do I can usually remove the second set of shims if I want. Shims are your friend. You can always use the above technique to allow you to insert a thicker single strip of hard wood in place of the shims.
Not long after a began building my own wood stands I got too old to work low down to the floor. So when I needed to raise the bottom level of a stand to raise the bottom tank off of the floor. I did this using 2x6 boards. I build a rectangle from them on end and then set the stand with its 4 inch long corner legs on top of the frame. The result was that the bottom glass of the under tank was then about 13 inches above the floor making it easier to work in and much easier to vacuum.
I was also able to shim the riser frame anywhere along the left and right sides as well as acriss the front anywhere I wanted/needed to do so.