thegasman2000, you might find this useful:
http://www.reefcentral.com/forums/sh...1#post24206611
It shows a hack of the MarsAqua I did. I don't know if your fixtures work the same way (I am inclined to think they are quite similar given the pin out posted above, the dimming may be reversed though). But either way, it gives an example of how to isolate the electronics and shows how to implement a higher resolution PWM (14 bit versus 8 bit) on the Arduino (assuming you choose a board and pin arrangement that supports this). It also includes code to fit the dimming to a bell curve if you are interested.
If your driver does indeed use an actual PWM signal to dim, then you can drop the RC filter on the output of the optoisolator in the circuit I posted in the thread.