Originally Posted by Hoppy
My tank is a low light tank now. I have used about 20 micromols of PAR for most of the past year, and algae is a very minor problem. For awhile I had it at 25-30 micromols of PAR, and was seeing a little more algae than I wanted to, near the top of the tank, which caused me to drop the intensity to about 20 micromols. This is with a LED light, and measured PAR with the Quantum meter, adjusting the LED driver current to lower the PAR. I think 25 is a better number for plant growth, but, with DIY CO2 the plants do well with 20. A lot depends on how much algae you are willing to accept - meaning how much maintenance is too much from your perspective.
But with the proper LEDs (proper as in correct spectrum, ideally peaking around 450nm and 680nm), 20 micromols of PAR will be much more effective than let's say 20 micromols of PAR from a full-spectrum bulb which from the spectral graphs I've seen tend to have huge spikes in the green-yellow range (which isn't completely wasted PAR, but it's definitely not as efficient for photosynthesis). So with an efficient LED fixture, 20 micromols PAR can easily put you in the high-light range.