too many variables. how do you arrange your rock ? Does your light have the option to on/off some bulbs or dimmable? Easiest way is to move the light up - starting with 6" ? And find the sweet spot. I am no expert though.

Yes, standard 90 gallon. 2 lights are on 12 hours, 4 others are on for 10 hours. Not dimmable. Rocks are arranged in a lattice wall (see picture)

https://im1.shutterfly.com/media/47a...D550/ry%3D400/.

In general, I think 12 hours is a bit much for lighting cycle. I prefer 7 or 8 for main lights.

Agreed. I went from a 6 bulb to a 8 bulb t5 and lowered the time period drastically, photosaturation is something alot of people don't consider