Ok new test... I took some pictures so you can see how i did it this time around... I looked and saw what i did yesterday and how gravity gave me such a high GPH flow rate...FYI that rate was measured from about 2" above the filters output.
Today the setup put output of the hose just above the tank open end no spray bar just the hose into the jug.
I am impressed with the number. I did it 3 times and while the fish hate me for it... each time i was within .36 seconds so i will call that my error and use the highest which was 29.06 the lowest was 28.69 with the last test was 28.88.
For sake of argument i will use the 29.06. I used a chronometer that we use at work when we measure ink viscosity with the zahn cup.
For sake of argument regarding gravity i shut the filter off and turned the valves on and no water flowed into the jug. I was higher then the tank but i tested it anyway so no-one can say i should have tested it

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With the filter on and the valve shut this is how i measured...
I turned the valve on and then started the chronometer, once the 5 gallon mark was reached on the jug i shut the valve off.
619.4 GPH is the flow rate. Remember this filter is clean and also has the polishing pad. In 4 weeks i will repeat this test to see how much flow i lost.