When testing the PAR output of our aquarium lighting systems, we often turn off water flow. This will give us a misleading result. With the water flow turned off, any sediment in the water will settle and the surface won’t be disrupted. The PAR readings will be higher than what our tanks experience in real life because of this. As water flow disturbs the surface, it makes it a little more difficult for light to penetrate into the water. Combining that with an upheaval of detritus and sediment, and you’ve got a real life situation that isn’t necessarily reflected by the PAR tests. To get a more accurate, real life result, try measuring PAR with the pumps on and see what happens.