Originally Posted by deadhorrorfan
I guess I'm just not getting it anymore with these new lighting systems (new since I last did planted tanks). I used to be able to rely on about 2 wpg with PC lighting. Now, with T5 and T5 HO I'm at a loss. What exactly is the difference and how do you tell what is "too much" or "too little" for your size aquarium. Lets assume the 55 gallon in question is going to be heavily planted with the aforementioned species and dosed with Seachem products if that helps. I really just need some sort of guidelines to help me make a purchasing decision I think....
First to explain some differences. The watts per gallon "guide" sort of works when one is considering "regular" fluorescent tube lighting and over moderate-sized tanks, say 50 to 125 gallon tanks. But even that still depends upon the type of plants, some need less light, some more, and just as you can't mix any fish together due to differing requirements, so with plants. Many planted tank aquarists don't remember this, and wonder why their high-light plants are falling apart in low lighting.
For the plants mentioned previously in your 55g, you have two options: a single tube T8 fixture, or a single tube T5 fixture. You could have a dual-tube T8 which would allow some more options in plants; in the meantime, reducing the light period can work. But remember that the nutrients have to be balanced, or the light will only cause excessive algae.
The T5 HO tubes produce approximately 1.5 times more light intensity than the same sized T8 tube of the same type. In other words, a 48-inch Life-Glo 2 tube in T5 HO will put out one and a half times more light than the same tube in T8. This may not seem like much, but it is. You have to compare equal types though, as the way a tube is manufactured, the type of phosphors used to produce the light spectrum, is different from tube to tube.
One has to basically forget watts because that is simply the measurement of energy a tube uses to produce the light it generates. And with modern technology working to make tubes that need less and less energy to produce the same intensity, watts means less and less in terms of light output. And this is where we simply experiment. I have used regular fluorescent tubes for over 20 years, so I know what to expect. Back then, the 4-foot tubes were all 40w, though they produced different types of light depending upon the phosphors, and while this impacts intensity somewhat, it was fairly consistent. But since then, they have become more energy-efficient too. The old original T12 fluorescents used 40 watts to produce "X" light, but the T8 tubes can use 32w to produce the same light intensity or even a bit more. The "T" number is simply the tube diameter in eighths of an inch, nothing more. But it so happens that more efficient light (more intensity for less energy) is produced by the smaller-diameter tubes. So as the tubes get thinner, from T12 to T8 to T5, they also produce more light for the same amount of energy so the energy needs (watts) can be reduced but the light intensity is still there. Generally.
T5 has also come out in NO and HO. Normal output, approximately comparable to T8 of the same size and type, and High Output. The NO is very hard to find, no stores around me carry them. And I believe there is an even brighter tube now, VHO (Very High Output) is what I think they call it. But one has to remember that these tubes produce much more light so you need less of them.
One thing to think of in all this is cost, not just initially but long-term. The T5 fixtures are more expensive up front. But so are the tubes. For me, a 48-inch Life-Glo in T5 HO is $50. In T8 it is $30. I can also use (as I do) the hardware store Phillips Deluxe Daylight which is only $4. And bearing in mind that these need replacing every 12-18 months...you can see quite a difference in cost. Now, undoubtedly manufacturers will continue to come out with new types, and prices of these may go down. But depending upon the life of the fixture, you might be replacing them in 10-15 years anyway.