Thanks for the reply. I`m interested how did you measure the micromols just from the distance and the fixture specs ? I measured it with PAR meter before and it definatelly wasn't 40, but 120. Again, I`m not sure how much is it now. But if it was 40 before, how would you explain the heavy trimming I was doing weekly ?
The PAR I referred to is that at the substrate level. Higher up in the tank the PAR is much higher with almost any light. There is some PAR data for the light you are using in Lighting an Aquarium with PAR instead of Watts I used that, plus the dimensions of the tank you are using, and the height of the light above the tank. I don't believe it is possible that you could get 120 PAR at the substrate and at that distance with that particular light.
Thanks Hoppy, that's quite interesting! Could it be that whatever PAR meter I borrowed wasn't working correctly and that I might actually have much lower light than I think I have ? Also I believe lower light can promote the growth of algae, because the plants can't grow fast enough to compete with them. Is my assumption correct ?