They will grow and utilize feed much better if exposed to light.
Continuous dark "is not" good for growth!
Many studies on this subject, and I have yet to find one that says continuous dark is good.
In my control system. I have analog inputs and outputs. The 0-10V inputs will have photo sensors that measure light intensity. The 0-10V outputs will control 6500K fluorescent lights with analog ballasts. This will allow me to program and control all aspects of light conditions.
I will also have photo sensors measuring and recording outdoor light conditions every 60 seconds 24/7. I can pick any day of the year and play it back inside the building, should I choose to. (We actually do this in customers office environments) Make the shortest day of the year outside into the longest day of the year inside. This is way beyond fish rearing, but it is all about productivity. You get the idea.
Obviously, no artificial lighting system can effectively duplicate the intensity of the sun.
Cecil should know that fish get spooked when you just flip on a light switch. An analog ballast is basically a dimmer ballast. Even the best ones have a "click to on" starting point using a dimmer switch. Because these usually use a dimmer switch, there has to be a point above 0V in the 0-10V output to make them work. My boss has created some circuitry in front of the ballasts (Phillips make the best ones) that eliminates this "click to on" point utilizing analog outputs from a PLC or other true analog device, so they truly go from 0-10V.
People who raise fish in buildings (people who want to increase profits) utilize light (photoperiod) manipulation as part of their overall protocol. It is not just for broodstock or hatchery phase, but overall optimization of final product output, with controlled inputs.
Hope I did not bore you
