Instructions
Make sure the skimmer box is working. In a working skimmer box aquarium, excess water raises the aquarium's water level to a point higher than the edge of the box. The excess water spills into the box and is siphoned out of the aquarium into a drainage box. The siphoning action usually doesn't rely on a pump or any kind of external power; it's a permanent effect of a correctly set-up pipe. If your pipe isn't siphoning, start it by inserting a length of tubing into the pipe from the outside of the aquarium, filling both the skimmer and drainage boxes with water, and sucking on the tubing until the pipe fills with water. Once it's full of water, it will pipe water forever as long as both ends of the pipe remain submerged.
Turn off your aquarium's pump. If you're planning to lower the water level, temporarily remove the fish, since you're about to drain some water.
Move the skimmer box up or down to change the water level. How exactly to do this depends on the type of box you have, but many models allow for the box to be adjusted in this way--check your aquarium or box's manual. Adjust the box so that its edge is at the level of water you want, or so that the desired level is about halfway up the box's opening. Turn on the pump.
How to Adjust the Water Level on a Skimmer Box Aquarium
A skimmer box is one way of maintaining the water level of an aquarium and of "skimming" the dirt and scum that accumulates on the surface. Once you understand how exactly the box works, and have made sure yours is functioning correctly, you can adjust the water level in your aquarium simply by moving the skimmer box up and down.