it should be low enough not to cause a break down of the GFO, yet strong enough to cause a nice tumble. If it has solidified, water can not pass through, I would change it.
Lester - it is actually not easy to get ALL of the GFO to tumble, most folks only usually get about the top inch or so to move about.
If the reactor starts to burp up big air bubbles through the GFO, then the flow is too strong and it will break apart and go into the tank. Slower is better