Since the essence of a hydroponic system is to bring an optimized level of water, nutrients, and dissolved oxygen to the roots of plants, it's necessary to keep the water moving. In a stationary body of water, the local uptake of nutrients by growing plants will quickly produce a zone where the roots no longer have access to the nutrients they need. With water constantly moving and naturally mixing, the nutrient levels will be constantly refreshed and renewed.
The size of the pump you'll need for your system will depend on which type of hydroponics model you use. Some types of hydroponics can use other methods to move water and may not require a pump at all. But most will require at least a low power pump to move water from the reservoir to the top of the channels, where gravity is employed to keep the water flowing downward to the exit. Pumps are graded according to the volume of water they can move per hour, so ask for help determining the minimum flow you need, then buy a size or two larger in case you decide to expand your operation.
