My own experience is that it takes close to 15 GPM/skimmer to ensure that debris makes it to the skimmer basket. At lower GPM, the debris tends to collect in the mouth of the skimmer but not make into the basket so anyone that starts swimming or the wind picks up (i.e. waves) tends to push the debris back out into the pool. Some skimmer manufactures give a 25 GPM minimum. Some venturi skimmers can go below 10 GPM.