Instructions
Check the overflow system in the sump for obstructions. It is common for fish or organic material to get stuck in the bend of a pipe in the overflow system, thus causing an uneven flow of water. Clear any obstruction you find.
Measure the size of the drill holes on each side of the tank. If they are not the same size, this could also cause uneven pressure. Contact an aquarium specialty store to discuss having the holes re-drilled, or purchase a new tank.
Calculate the flow rate for the overflow system and see if it matches the output for the aquarium pump. A standard 1-inch drain line has a flow rate of 300 gallons per hour when positioned at the top of a tank. A drain positioned at the bottom of the tank has the flow increased due to gravity and volume to as much as 600 gph. The output listed on your pump does not factor in losses due to pumping against gravity. So, if you calculated your overflow to 300 gph and bought a pump that maxes out at 300 gph, but pumps water up a 4-foot pipe, it will underperform and cause uneven pressure. Replace the pump with a stronger model.
How to Even the Pressure in an Aquarium Sump
Aquarium owners often install sumps to help regulate the environment in the aquarium. The additional water stored in a sump acts a reservoir and aquarium owners can use it to introduce additives into the main aquarium. Sumps also keep water levels flowing and constant, unless there is a problem with the pressure. If you want to even the pressure in an aquarium sump, you need to engage in troubleshooting, as there are multiple causes of uneven pressure.