Join Date: Jul 2010
Location: Outside Philadelphia, PA
First, it is critical to get the tank level. If you don't, there will be a lot of stress on the silicone sealer, and long term, the tank will leak.
Since you have a lot of shimming to do, and they seem to be rather thick, I recommend that you got to Home Depot or Lowes and get a bundle of shims. They are about the size of paint stirrer sticks, but they are tapered, with one end being about 1/4" and the other being as thin as possible. They are very inexpensive.
You install this type of shim in pairs one over the other in opposite directions, you can them tap them with a hammer to make the place you want to shim thicker. This lets you make variable thickness shims all around the base of the tank, from almost nothing to almost 1/2" thick. If you need more you can put a scrap of plywood or similar material under the shims.
You will have the ends of the shims sticking out from the base of the tank. Once you get them all correct, you can easily trim them off with a utility knife.