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.