Quote Originally Posted by Brhino
fluorescent lights cannot be dimmed, they're either on or off... although if you have multiple bulbs, depending on the fixture, you may be able to turn some off and leave others on.
Your other option would be to install some LED lighting to use when you only want a little light.
Fluorescent lights can be dimmed with the proper dimming ballast. They run from about $60 - $275. depending on the number of lamps, voltage, and wattage of the lamps.

LED light fixtures using a dimming driver can be dimmed manually or automated with something like an Arduino.

For the short term, add some floaters to the tank and let the shrimp find their own comfort zone.