If the terrarium is lit only by the 18" tube or tubes, the plants are
certatinly not getting enough light. Nepenthes are plants of the tropics that
inhabit forest edges, and other mostly open areas. When grown in the northern
temperate zone, they need the brightest light in winter. While flourescent
lights might seem bright to our eyes, to a plant they are not very. I have
always grown plants under lights and over the years have come to realize that
I really could not give my plants too much light. In my growing areas there
are two four foot tubes over every eight inches ( 19 cm. ) of growing area
width, and over an area 27" wide I have eight four foot tubes.
Longer tubes are more effiecient at making light than shorter ones, so two
24" tubes do not equal the light output of a single 48" tube. At each end of
the tube the output of light drops off and so and having tubes that hang over
the ends of your terrarium would be better.
The plants should be as close as possible to the light source, their tops
can be about 4 - 6 " under the tubes. Any surfaces in the terrarium that can
be, ought to be painted with white paint to reflect light or coated with
reflective mylar.
You can help to make up for lower intensity light by increasing the duration
of light during the day; you may want to keep the light on for fourteen to
sixteen hours each day and while this isn't normal, plants seen to do better
than with shorter days. Flowering in many plants will be affected by the long
days, but don't worry about it for the time being.
With age, the tubes loose a significant amount of their light output; I
replace the Gro-Lux tubes every year. You may want to use a permanent marking
pen to write on the end of each tube the date that you install the tube. This
helps remind you when to replace them as they still will appear to be
working, but their light will have diminished.
Vita-Lite tubes are brighter and last longer - they are guaranteed to have
most of their light output for two years, so they need replacement less
often. I find plants have good color and flower well under a mix of two
Vita-Lite tubes to one Gro-Lux wide spectrum.
Other factors to consider are anything that may decrease the light reaching
the plants, such as dust on the tubes themselves, glass and plastic barriers,
and foliage in the way.
I hope some of these ideas help get your plants back to health,
Good Growing,
David