Ensuring that the aquarium is level is important. Sometimes if it is slightly out of level it doesn’t matter as the water surface is often above the lowest part of the aquarium hood, or there is a black band on the aquarium to hide the water surface. If the water level can be seen it needs to be visually correct, and in any case it is best if the stand is level in any circumstance for support.
There are two approaches to levelling, and this depends on the start point of the construction – is the stand being constructed from the start or has the stand been purchased complete.
If the aquarium stand is being constructed, then consideration should be given to the site it is to be used in. Using string or similar, mark accurate lines exactly where the stand will go. Then, using a reasonably long bubble level, lay the level carefully along the string, first along the front edge and then along the end line. (If the level is not as long as the line, use a piece of wood the length of the line and put the level on that. Ensure that the wood is absolutely straight, and not warped at all.) Note in each case if the bubble is exactly in the middle. If it is not, raise the level until it is. A measurement can now be taken of the distance that the level had to be lifted to achieve true horizontal. The same should be done with the side lines.
If the levels of the floor are true, then no problem. If not, then proceed in one of two ways. When constructing the stand, allow the additional measurement to be added to the height of the stand at the necessary end, ensuring that strength and stability are maintained. Properly done, the stand top will be level. Or, second, construct the stand as normal as though the floor was level. Then construct a filler, the thickness of which should be the amount by which the level had to be raised to make it truly horizontal.
The downside of adjusting the stand itself is that if it were to be moved to a new location, then it is not going to be level, and fillers are going to be needed. On this basis, it is best to construct the stand normally, and use fillers as described above. If inserting a filler, make it so that support is given to as much of the stand as possible. It may be necessary, if a floor slopes, to construct a triangular shaped filler.
If the stand is ready built, then use fillers where necessary as described above, after checking with a bubble level in the same way.
Aquariums full of water and rocks are very heavy and need to be stable. This is one of the reasons why the stand is levelled, as well as it being visually correct. Maximum and effective support is required if the floor area is out of true. Hopefully, most floors will be level. A little off topic, make sure that the floor itself is strong enough to support the aquarium. If in any doubt, seek qualified advice.