Picking the right water pump for an aquarium may not always be an easy task. You have to consider the size of the tank, type of equipment being used in the sump, the amount of outlets there will on the plumbing to feed media reactors, the size of the holes in the aquarium, and the vertical distance the water will have to travel before getting back into the aquarium. What most hobbyist do is use a much larger water pump than is required and simply use a valve to reduce water flow. Using valves is a good way to adjust flow output to match what your aquarium system requires, but closing the valves too much may lead to pressure issues down the road. Some pumps are designed to handle these stresses, but others are not. Additionally, the plumbing has its limitations as well, though its rarely an issue outside the occasional pinhole leak or leaky connection.