In alt.aquaria James Smith James wrote:
Has anyone used T5 in a freshwater tank? is it really more effective than
standard tubes?
Does anyone know if the watts rules still apply to T5? and how much light
(in T5 terms) should I be looking at acheiving? Would a single T5 tube be
enough? or should i be looking at power compact t5 (55w) or even 2xTubes?

As with all rules of thumb, the watts per gallon is just a general
guideline and not an absolute rule. Your first priority should be to see
if appropriate bulb spectrums are available in T5 size for your tank
length in your area. You want bulbs with strong peaks in the red and blue
spectrums for maximum photosynthesis (key parts of the photosynthesis
process rely on specific red wavelengths, blue is a supporting spectrum).
Some people compensate for lack of spectrum peaks by throwing more watts
at the tank, but I prefer to chose the bulbs wisely. If you can't find the
right spectrum in a T5 bulb, but you could in say a T8 bulb, then it would
be more effective to go with the T8 and maximize the lighting in the right
spectrum rather than throw more watts at it.

It really does come down to the ballast and bulb regardless of what system
you're using. The ballast affects the intensity and the bulbs affect the
spectrum of light available for the plants. If you can find plant
appropriate spectrum in T5 bulbs, you'll probably still benefit from a two
bulb system, each fixture having a seperate brand/model, in order to
balance a pleasant viewing color with the proper spectrum for plant
growth. For example, on my 36"x18"x12" tank, I have a 3xT8 fixture. Each
bulb is a different model. Two are specifically chosen for having a strong
red peak in their spectrums (ZooMed FloraSun and GE Freshwater). The third
is a more general bulb (All-Glass generic) used for color balancing for my
viewing of the tank. The tank still has a slight purple cast to it, but
not too bad. This tank where I took specific care to chose two strong red
spectrum bulbs has far better plant growth than my other two planted tanks
where I had focused more on watts per gallon when setting them up.