If one or more glass panels in your aquarium are not perfectly flat, this can cause the aquarium to appear skewed. This is often due to the way the aquarium was manufactured or due to damage during transport. If you notice that your aquarium glass is uneven, you should contact the store you purchased it from to see if it can be replaced.
Uneven substrate
Make sure that the aquarium substrate is level and well-distributed throughout the tank. This will ensure that the water pressure is evenly distributed and the aquarium is stable. Be careful not to overdo it with the substrate; too much substrate can block the water flow around the roots of the aquatic plants and lead to root rot.
Uneven aquarium stand
Make sure that the aquarium stand is level. If the stand is not level, this can cause the aquarium to appear skewed and can lead to the tank developing leaks over time. You can use a spirit level to check if the stand is level. If it is not level, you can shim it with shims to level it out.
Uneven flooring
If your flooring is not level, this can also cause the aquarium to appear skewed. You can use a spirit level to check if the flooring is level. If it is not level, you can level it out by pouring self-leveling compound over the floor and spreading it out evenly. Once the compound has set, you can place your aquarium back on the floor.