The higher you raise the light the larger the circle of light gets, so the lower the light per unit area gets. PAR is light per unit area. If you want to be sure to have low enough light, just raise it 2 inches instead of one inch. Then, if you still have difficulties with BBA you can either dose Excel at 2X the recommended dosage - or 2 ml per 10 gallons every day - or you can raise it another 2 inches. Without a PAR meter you can only do this by trial and error.
After 4weeks small tufts of BBA are starting to show up on dying leaves at the bottom of tank, dwarf sag, and on the anubias petite. I removed as much as I could and raise raise the light 1 inch.
Now my question is will a 10.5 inch fixture just extend the lighting footprint outwards or does it alter the available par values enough to warrant raising the light? The 8.5in is giving a spotlight effect with plants growing up and inwards.
The algae that you want to look for is black brush algae, BBA, which will be black tufts on non-growing leaves, and on hardscape. As soon as you see any, remove it, and raise the light an inch or so. Keep doing that until either you see none, or the rate at which you see it is easy to take care of.
Okay thanks for your help! I'll start there and tweak the height... what kind of things should I be looking for that would tell me to raise it? How quickly can algae bloom? This will be first tank in about 20years, so I just don't remember
I used the data from i4x4nMore, which I plotted on log-log graph paper to make extrapolating easier. I suggest starting with the light at the top of the tank, but being ready to raise it a couple of inches if necessary. I stopped making PAR meters, from boredom. I'm still looking into the possibilities of using the process described in http://www.plantedtank.net/forums/sh...d.php?t=453225 to make more of them, with less boredom.
Sorry, I don't mean to to be a pest, but I'm trying figure out how high to hang the light to achieve upper low light par. At what height do you think it would be 40par? Which one of your charts did you use to get get to that answer? Can I assume the lower numbers chart you made and the one i4x4nMore or more realistic? I just need a ball park so I have somewhere to start from and tweak... are still selling par meters?
thanks for the reply. in my case i want low light, and my tank dimensions are 13x13in surface area, so i currently have a 23w daylight in a 8.5in dome and the bulb does NOT stick out... in the mean time, based on your previous experiments, which chart should i be using?
When there was big disagreement about how much PAR one of those bulbs would give, there were 3-4 of us making measurements, and making various errors in doing so. Plus, we found that a bigger CFL bulb that would stick out of the dome reflector would give significantly less PAR, and we were mostly using 8.5 inch domes, not 10 inch ones. With the 10 inch dome the bulb doesn't stick out, as I recall, but with the 8.5 inch one it does. I haven't revisited that subject since then, but I have planned to do so sometime.