In alt.aquaria George Pontis wrote:
A question about fluorescent tubes for use in a Marineland Eclipse3 hood. The
lamps supplied with the hood are Eclipse Natural Daylight F18T8 24". This is over
a planted, community 37G tank, 22" high.

Well, the description of the bulb tells you the parameters you will
require in the hood. F18T8 tells you that it takes a 18W T8 bulb (24").
This should be similar in output to a 20W T12 (T12 are larger diameter
bulbs that might not fit in your fixture). There are many T8 bulbs made
for the aquarium, so it's a matter of what you are looking for in terms of
color. For good plant growth, I'd hate to say it but you'll need more
wattage than that single fixture. Otherwise use only low light plants such
as java ferns.

Some good single bulb T8 plant lights are Zoo Med's FloraSun and GE Aqua
Rays Freshwater (not Fresh & Salt, that's a different bulb). Actually, I'm
not sure if the GE comes in T8 at 24", but I know the FloraSun does. It
would be around $10 for the bulb probably. Philips also makes a really
nice daylight bulb, Natural Sunshine, but again I am not sure if it comes
in 24" T8 or not. Otherwise, just look around online at various mail order
pet supply companies and see what they have in T8 24" bulbs. You might
also try the lighting aisle at the local hardware stores.