It isn’t just evaporation that can cause the return section of your sump to run a little low on water. Sure, it’s the main cause, but the slightest buildup of algae in the teeth of the overflow can cause the water level in the tank to rise, thereby decreasing the amount of water in the return section. Additionally, buildup of coralline algae, sponges, and other organic gunk can cause an identical effect. Besides causing the pump to run dry, this sort of blockage could also cause an overflow of the aquarium if things are not designed properly, though it would be hard to do. For this reason, it is crucial to clean the teeth of your overflow box, as well as any other overflow teeth located through the system, such as those in a refugium or similar space. It’s usually a simple task and can be easily knocked out during a water change or just a little hands on with the aquarium.