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when people say 50 micromols per sq m per sec. is good enough to grow plants slowly. Does that mean it's 50 micromols per sq m per sec. at the very bottom on the tank? I'm assuming yes but i just want to make sure.

I was able to borrow a par meter and i finally got my readings to mid 30s to low 40s at the very bottom. Closest to the surface reads around the 50s. I'm growing african ferns, anubias, moss, crypts and blyxa japonica for now. Will that do?
 

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Yes, along the bottom of the tank.
Some plants obviously are taller and will be much closer and have higher lighting.

Others not so much.

Regards,
Tom Barr
 

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Unless the light is hanging pretty high above the tank it is very hard to have 50 micromols at the substrate without having 150 or more at the water surface. Even if the bulb is 2X the height of the tank above the tank, the intensity at the water surface will be about 2 times that at the substrate. Another way to get around this is to have more than one bulb, and have them widely separated. That way no point in the tank is really close to both bulbs at once. Plants in the tank will of course shade the lower parts of the tank, changing the intensity even more.

Another way to reduce the variation in intensity for at least part of the tank is to keep the glass very clean at all times. Then light reflected off the glass increases the intensity at the lowest levels in the tank.
 
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