Lighting - acrylic tank

Hiya
I have an acrylic tank (about 80 litres) which I am using for three
voluptuous goldfish after the sad demise of my lizard.
The tank is unlit, and due to the size of the fish I have it filled almost
to the brim for them. I do not see how I can use lighting which rests on
top of the tank due to the height of the water, and am wondering if I can
use a side or rear hanging light instead? My poor plants would really
appreciate it, but I have no idea whether it is feasible. I was thinking of
a desk lamp angled into the tank with a good bulb, but what would be the
type of bulb to use without damaging the acrylic?
Cheers

if you buy a glass hinged lid you can set the lights on it. it's what
most tanks come with. since you are trading from lizard setup, you
wouldn't have it, but they are relatively cheap and come in a variety
of lenghts. also, fish need a lid to keep them from jumping out of
the tank