Pachira Aquatica - leaves dropping and my grow-lamp

Hello,

This is a question regarding my Pachira Aquatica tree - see the
following pic :
http://picasaweb.google.co.uk/tjrund...10226768198770. I
got it back in June, and it's been shedding leaves ever since. In the
summer months it's wasn't too bad after it stabilized, but then in the
recent months it's been loosing a branch of 6 leaves a week -
sometimes 2 per week.

Sadly it's becoming a former glory of itself, which is shame cause
it's a really cool talking point in my flat, and I love it. So much so
that I thought some additional light might help. Remembering my
friend's from university 'grow up', I bought a sodium floodlight which
is on from 4am to 7am, and then 9am through to 6pm.

I did some cursory research after wards and read that halide lights are
better than sodium lights as plants have a better reaction - I was
always taught that orange was the best colour because it was absorbed
most by green leaves. However it seems that the colour temperature has
a bearing on the efficiency of the photosynthesis...

My flood light's about 75W - is that strong enough, or is it not going
to make any difference? Would a 150W halide bulb work better? Since I
installed the lamp about 3 weeks ago, I've not noticed any reduction
in the rate of drop-off.

I think I'm sadly resigned to the fact that my flat is just too dark
(no direct sunlight except in Summer for an hour) to support such a
plant, even though they are advertised as being low light plants.

I don't think the leaf drop-off is due to over / under watering - the
planter has a bed of gravel and I keep that wet to aid humidity, and I
spray the plant every week or so (I travel a lot so I'm not actually
here that much). I let the top of the soil dry before watering - again
- typically every week. The flat's warm all year round.

Any advice?

Thanks in advance,

Tom.