My practice has been to deal with leveling before I even beign adding any hardscape. I almost always need to level tanks. The only two that have been perfectly level from day one are my inwall tanks where I had a carpenter do all the work. However, I also make alot of my own stands. racks using wood.
So here is how I do it, for whatever that is worth.
I build the stands to be level, but the nature of the solid slab that are the floors is not even and level. Next, I use pressure trated lumber which has a tendency not to be straight. I have to level it all. So, I cut precciesly, use multiple screws and use come-alongs to counter bending during assembly. Despit this or even using commercial iron/metal stands, things are usually level when I start the next step where things become uneven.
I start to add water to the empty tank. The goal is to increase the weight which should reveal any leveling issues. I also use a fairly long level. In addition I use my eyes and a ruler. If a tank is level then the water level inside the tank should also be even. If if is clearly off Imay be able to see this but I may need to measure or use the level to check. The goal of this is to start shimming the stand. I should mention that none of my stands are on carpet. They are on wood floors or linoleum floors.
Using shims works find as long as they can be put into place under the stand. So, it is easier to get the shim in place where they should be the less there is in the tank. Too much weigh and I cannot even hammer them into place. And this is why I add the water gradually during the process. Sometime I actually have to remove some of the water to get the shims in place.
There is one other trick I use that is akin to styro under the tank. All of my tanks have plastic frames on the bottom and top. I have no frameless tanks. So I see no point using styro since it is only the frame which supports the tank on a stand. Most, but not all, of my stands are frame only support. The trick is I use foam insulation tape aka weather stripping. This is used to seal doors and windows. I get the type which is self adhesive on one side.
The tape comes in a variety of widths and thicknesses. On my wooden stands I am placing the tank frame to rest on 2x4s on edge. But 2xx4s either regular or pressure treated tend to be uneven as well as warped. But, even if fairly straigt, the surface is definitely no even. The weatherstripping acts as a self leveler. If I did rimless tanks i would have a full plywood top on a stand and would use styro.
Now, here is one more secret re leveling. Almost no tank needs to be absolutely level. In fact, even a tank that starts life perfectly level may not remain that way. So, my goal with leveling tanks is not perfection, instead it is minimization. Levels have those lines for centering and the goal is to keep the bubble between those lines. My goals is to insure that the bubble does not touch the line on either side of a perfectly centered bubble. I will accept a not 100% centered as long as it starts and stays between the two lines over time.
Over the years I had only a 2.5 gal tank actually crack the glass. I did something stupid with it. I left it hanging half over the end of a table and filled it about 1/3 with water and then went to do something else. When I came back the glass had cracked. I have also had 3 tanks leak. All 3 leaked without any cracking or breakage meaning the seal gave. One of the 3 was a new 20L and I just replaced it. I had gotten it so cheaply that it made more sense to do that than to deal with a getting a refund.
Here is a pic of my 150 gal being placed on the stand I built. If you look closely where the tank sits on the stand you will notice a black line between them, it is the weather stripping.
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Here are both stand and tanks on them and you can just make out the weather stripping under the 125 (it has a black frame).
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As you can see I also paint my wooden stands. I use a semi-gloss exterior paint for this. Stands will get wet repeatedly over time. I never finished those stands. They were designed to have a finished look if I wanted to do so. Instead the 125 has a 40L under it and the 150 has supplies and a canister filter under it.
Those tanks were both purchased used. They have been up and running for several years ( I think about 5 -7).
