The same thing happened to me. I leveled my Petco steel stand before I filled up the tank, but it's still not level because of the floor in my old apartment building. There is about a 1/4 inch difference in water level from front-to-back of my bank.
I started the set up and now have a rock area for my dwarf cichlids, several crypts, some vals and will add a few more plants tomorrow. I have almost the entire tank filled up (by buckets) and will finish that also on Tuesday.
The problem with the internet is that it can have both good and bad information. This guy knows nothing about the correct way to use shims!
He starts off with the idea that shimming on carpet is easier than on solid floors as you don't have to worry about scratching. DUH! Does he not care if the shim snags a carpet fiber and pulls a big runner? Depending on the type of weave, carpeting can be pulled and need repair. Far better to put something thin between the shim and the carpeting. Aluminum foil can work.
Second problem is that he doesn't look at what using a single shim does for the weight. If you put a single tapered shim under a board, the thick side will carry most of the weight and either compress or tend to twist the board it supports.
This drawing shows the issue:
If the shim compresses, you have to level a second time. If the board twists, you put extra, unneeded stress on the stand. To keep the weight supported by the full width of the supported board rather than just the edge, shims should always be doubled. One from each side so that the shim top is level.
Shims should be put under the corners of the stand where the weight is supported by the upright wood sections, not at the centers of the horizontal boards.
The shims should not be simply broken off. When this is done the wood grain will often tear some of the wood out from under the stand and fail quicker. The shim should be scored with a sharp knife before it is broken off.