Don’t have a way to measure exact flow rate. Pumps were a 325gph on the 15W and a MJ600 on the 9. 3/4 max on the 15 & full on the 9.
That should have been enough flow for the 15W to work. They only other thing I can think of is to try a black or lights off for a few days. The window question is a good one too.