In alt.aquaria James Smith James wrote:
I already have a "correct spectrum" T8 -
http://www.arcadia-uk.com/en/220freshwaterlamp.html so the problems requires
more watts rather than a better spectrum.

Hate to break it to you, but that is not a correct spectrum bulb. It
completely lacks a spike around 680-700nm (closest appears to be 620nm or
so), which is essential for photosynthesis. There are two key types of
chlorophyll which require 680-700nm wavelengths to activate.

Unfortunately, this is an extremely common problem with fluorescent bulbs,
even bulbs that proclaim to promote plant growth. I assume this is because
replicating this portion of the spectrum is difficult in fluorescent
technology. I researched bulb spectra for weeks before I found a few that
came even close to providing the required red peaks. The GE Freshwater
(not Fresh and Salt which is a completely different bulb) and Zoo Med
FloraSun that I chose for my T8 system came closest with having peaks
around 650-660nm and a decent taper off in the 680-700nm range.

There are several options a Interpet Triplus T5 Power Compact Lamps:
a.. Freshwater aquariums to stimulate lush plant growth

Super bright daylight-balanced aquarium lamp
* Freshwater aquariums to stimulate lush plant growth

I can't seem to find the spectrum of that bulb with a Google search,
perhaps because Interpet was bought out this spring, so finding
information on any of their products online is not easy these days (I
remember having the same issues searching for information on another of
their products a couple months ago). If you are looking at a bulb in the
store, see if the packaging has a diagram of the spectrum and check for
peaks in the 650nm+ range.

Or in a regular T5:

As above or:
Arcadia T5 Plant Pro Lamp
a.. Superior Plant growth
a.. Enhances colours of fish and plants
a.. Satisfies plant species with a higher light requirement eg Lilleopsis
(http://www.aquatics-online.co.uk/lis...=42&rets=1 01)

This bulb does have a nice peak in the 650nm+ range. It looks like a good
prospect. Notice, compared to the other Arcadia T8 bulb above, that this
has more than one peak in the 600-700nm range. The T8 bulb just had that
first peak right after 600nm. This T5 bulb has two additional peaks. It
does trail off quite a bit heading toward 700nm, but that is pretty
typical in fluorescents. The basic thing to look for when comparing those
two bulbs is that the T5 has more red spectrum while the T8 has more blue
spectrum (which is typical in a bulb that's meant more for viewing or a
fresh/salt bulb).

I too would like two tubes - but one is a lot easier to get inside the
exiting hood. I will have another look!

As I said before though, you might desire another bulb just to make the
viewing colors less purple (as many plant bulbs give a purple cast). It
doesn't even have to be another of the same length/wattage. If you could
squeeze in even an 8W T5 "soft white" bulb in the hood (say by mounting
ballasts on the outside back of the hood), that would likely be enough to
balance out the colors for viewing. Could also be that the plant bulb is
sufficiently "broad spectrum" to not have the purple cast, in which case
you wouldn't need a color balancing bulb. If you can find a CRI (color
rendering index, how close to real sunlight does it render colors) rating,
the closer to 90 that is, the less likely there is to be a significant
color difference in the light.