Quote Originally Posted by samtheman
if I adjust my third growbed to fill and drain within 16 minutes then I get no water going back to the fish tank at all. My question is : is it good or bad to fill my third growbed in 35 minutes or is that harmful in some way?
No set answer for that Sam....'generally', I would think once a plant is established, 35 mins wouldn't be that big of a deal. You'll have to keep an eye on the plants.
Maybe you could slow the flow into the 2, that are filling quicker ...sending more to the 3rd and to the FT (?)

Your 100 gal GB's 'should be' holding aprox 40 gallons of water each, due to the displacement of your media.
If my math is correct... ...you would need 160 gph going into each bed, to fill in 15 mins. This would leave no flow for the fish tank....considering, your pump's performance chart was showing 480 gph at 5 feet of head.

Sorry Sam.....if playing with the flow to the grow beds doesnt work, you may need a second (smaller) pump to send water just to the FT. If you get a 2nd pump, consider adding a spray bar to it.