Most (perhaps all?) life runs to a light and dark rhythm. Humans certainly do, we need our sleep.
Life on the wild reef isn’t an exception. During the day many fish and other life forms are out and about finding food. As soon as darkness approaches they find a safe sanctuary where they can pass the night, as at night other life emerges, much of it predatory.
So life is tuned to a night/day cycle. So it is in the aquarium as the livestock need the cycle to function. I don’t know what would occur to daylight loving stock if the lights were left permanently on, or vice versa, but it isn’t an experiment I’d care to undertake.
Providing a day/night cycle is simple. Let’s start with a fish only system. This system type is not normally lit by metal halide bulbs, but by fluorescent tubes. For the cycle to work there needs to be at least two tubes, one actinic and one marine white. There also needs to be two electric timers.
Each tube is connected through its ballast to a timer. The timers are set so that the actinic tube switches on about ½ hour before the marine white. At the other end of the cycle, the marine white is set to switch off ½ hour before the actinic. The marine white can be left to run between 8 and 10 hours, meaning that the actinic tube will be on 9 to 11 hours. The fish will now wake up to a ’dawn’ and be stimulated to prepare for night at ’dusk’.
The reef system is very similar, except that lighting on a reef system is more critical and more lights will often be present. This does not make any difference to the lighting cycle system though. If a metal halide is in use, then an electric timer will be required for this. The metal halide is the main light for the day period. If marine white tubes are in use, they will need to be connected through one electric timer. If not already present, the aquarist will need to provide one or better two actinic tubes, which need to be wired through the other electric timer. Again, the actinics switch on and off ½ hour before and after the main lighting. The main lighting can again be left on between 8 and 10 hours, again meaning the actinic will be on for between 9 and 11 hours.
The lighting periods suggested above are not critical, and the aquarist can make the timing choice according to the needs of the livestock.
Sometimes it is recommended that the main marine lighting should be on for 12 hours. This recommendation relates to the average tropical day which is circa 12 hours. However, there isn’t a need for intense lighting for this length of time as it doesn’t occur in the tropics. Either side of midday the light is intense, but earlier and later it is less. In addition, the power of the sun cannot be equated in the aquarium.
If a lighting regime such as outlined above is implemented, the aquarist will perhaps be surprised at how quickly the fish ‘learn’, appearing a little after ’dawn’ and preparing for night at ’dusk’.
The lighting regime is also good as it is much more natural than plunging the aquarium into light or darkness, which is bad. The closer to nature’s dawn/dusk environment the system is the more contented the livestock will be.
There is an advantage to the aquarist too. Watching the fish react to lighting changes is one. Using a torch and watching the night life is another, though more for the reef system. The aquarist could be surprised at the amount of life and movement there is after dark.