My 20g is my high light tank, with 2x20w NO bulbs w/reflectors and 3x13w power compacts. That gives me 3.95 wpg over this tank.I was looking at another thread in this forum where I see that Carlos adds 21ml of Flourish and 21ml of Flourish Iron weekly to his 20 gal tank! This seems like an awful lot to me. Carlos, is this a high light tank? I am curious how you came to these numbers? I assume you worked your way up to them and compared your growth rates, etc.
I gradually worked my way up to those levels through trial and error. For high light tanks, I believe that high iron/traces are crucial for plant health. I did very little comparison of growth rates, actually. I judged everything solely on plant health. The extra iron/traces doesn't make my plants grow faster, but it sure makes them look much, much healthier and greener! Growth is fuller and more robust to my eye.
To work my way up in dosing, I basically add a low amount in the beginning and gradually increase the amount every week until I no longer see any improvement. One week, I was dosing 30 mL of each weekly. I didn't see any new algae growth, but I didn't see any change in plant health so I dropped it back down.
My 55g (which has 2.55 wpg) doesn't need nearly as much iron/trace dosing. 12 mL weekly of Flourish and another 12mL of Flourish Iron seems plenty. It all depends on the tank, and the lighting levels.