lighting

I've just moved my two fantails to a new, bigger tank. All is fine - I spent
a long time getting it established etc etc and water is fine. However the
old small tank just had one of those pygmy lights in it - the kind you get
inside ovens etc? This one has the full fluorescent tube - and they hate it.
I have live plants so need to have the light on but when it's on, they hide
in the corner under the filter although they swim around fine when it's off.
i'm in the UK and at our local aquarium shop, these are the standard
fluorescent tubes sold for tanks so I'm not really sure how to alter it. I
thought maybe I could get a dimmer switch that is on a plug socket but
certainly no electrical shop around here seems to have one.
Or will the fish get used to it do you think?
Thanks for any help