Originally Posted by jusfishinaround

I picked up a simple shop type light and added 2 32W T8 5,000 K bulbs. Now here's the question. The box that the shop light came in indicates that because the light was designed to work most efficiently with the T8 bulbs, each bulb would put out 30% more light than an equivalent T12 40W bulb. Running the math in my head says 30% more than 40 watts is 52 watts. Soooooo, two bulbs in that fixture means I am getting about 104 watts of light or about or almost 2 watts per gallon for my 55 gallon tank. Is my reasoning correct??

Originally Posted by mathas

Not necessarily, because light isn't measured in watts. In general, a 40W fixture just means the fixture uses 40W of electricity at any given point in time. A 40W bulb means it can handle 40W of power without being damaged. You can't really tell how much light is being output based solely on how much power is being used.

How much actually light gets directed into the tank is dependent upon the bulb's luminous efficacy (which, in turn, depends on what type of bulb it is... T5, T12, CF, etc.) and the fixture's reflector quality (or lack thereof). A 54W T5HO bulb with a good reflector will put far more light into the tank than 54W of incandescent lighting, but both would still give just under 1WPG on a 55g tank. This is just one of the many reasons that WPG is a rather overrated metric in my opinion.

Are you getting more light into the tank with your T8 bulbs that you would with T12? Almost definitely. Do you have "almost 2 watts per gallon"? No, you have 1.16WPG.

Originally Posted by jusfishinaround

Mathas - so if I am following your logic, then can it also be stated that just because I do not have more than "1.16" watts does not mean I couldn't keep a plant that requires more light? I remember seeing a comment made somewhere on some website about someone saying that with bulbs getting more and more efficient over the years, he/she found that they were using fewer # of watts to accomplish the same objective with their plants.

Originally Posted by mathas

In my opinion, this is correct. Note that you said 'just because I do not have more than "1.16" watts does not mean I couldn't keep a plant that requires more light."

When you are talking about a plant that requires more than 1.16 watts, who was determining that requirement? What type of lighting were they using? How deep was their tank? Where there other surface plants to block out the light? How good are their reflectors? How similar are any of those factors to your own situation?

There are just too many variables involved to arbitrarily state that if someone's lighting fixtures uses x watts of power, that they're limited to plants of type u, v, y, and z, at least not without knowing a lot of other information.

Now that's not to say that you can keep the most light-demanding plants you can find in a 55g with only a single 5W compact fluorescent bulb, some common sense is necessary, but some leeway is certainly present in any rule you're likely to find.