There are many other factors in addition to he wpg
, such as the depth of the tank, how high you mount the light, the efficiency of the reflectors, and how long the light is on. So, no need to panic. You might be pushing 3 wpg
, but you can still do OK without CO2
. It is a matter of balancing out your fertilizer & the lighting period & the number of plants you have.
The main problem with too high light is algae growth. You can limit that by having lots of plants (and some fast growing stem plants (at least until the slow ones grow in), and giving the plants optimal growth conditions. <That usually means adding some ferts, but it is easy to do.>
You can also play with the photo period, maybe breaking it up as others had suggested. If you have to decrease the light level, some fixture will allow you to light just one bulb. It all depends on what ballast is in the fixture & how it is wired. <Take out one bulb & see if it will still light.> If all else fails, you can always swap in an actinic bulb to decrease the light that way.