There is only one thing that is a little more important than lighting with a marine aquarium and that is water quality. Lighting is only really important though when light demanding life is kept, such as in a reef aquarium.
If the system is fish only then lighting is important but nowhere near as much. Fish need to be able to see and the aquarist wishes to see them. That’s it. Of course, it is still desirable to have a dawn/dusk effect using at least two fluorescents, usually blue and white, with electric timers so that something approximating the increase and decrease of light at the beginning and end of the day can be provided. When should the fluorescent tubes be changed? If a tube is seen to be flickering then it needs changing. If the aquarist thinks the light has diminished it probably has, so get a replacement tube. If the ends of the tube(s) are blackened then it is getting old and could do with changing.
The reef system is a different matter. Here, lighting is usually fluorescent tubes, metal halide or a mixture of the two. As corals are being kept, be they soft or hard types, or a mixture, the lighting needs to provide the correct spectrum and intensity. When the lights are new they will do, but as time progresses the lights reduce in intensity and the spectrum can shift, both undesirable. The only way to correct these problems is to renew the light source.
When to renew is the subject of argument among experienced aquarists and particularly among very keen ones. Some argue that fluorescents should be changed as frequently as every 3 months and metal halide bulbs every 9 months. However, the guidelines for changing tubes and bulbs are more generous than that.
In the case of fluorescent tubes the point when flickering and blackened ends arises as previously mentioned should never be reached. The tubes should be changed well before that. The guideline is 12 months maximum from the date of first use. There isn’t a problem in changing the tubes more regularly than this of course and will not do any harm at all. I change my fluorescent tube array every 9 months.
Metal halide bulbs could be changed after a maximum 24 months from date of first use. This seems a long period and many aquarists, to be on the safe side, change more frequently. I do not use metal halide, but if I did I would probably change every 12 months. The expense is not too high. Changing the bulbs varies with different aquarists, some change at either 9, 12, 18 or 24 months! Just be safe by changing regularly at a shorter interval than recommended.
The recommendations for the expected life of a bulb, particularly metal halide, are usually available from the manufacturer. This is often given in hours so that varying lighting ‘time on’ periods can be accommodated. Once the recommended hours are known it is a simple matter to divide this by the daily running time so that life expectancy can be seen.
All aspects of marine aquarium husbandry require attention from time to time. It is essential to pay heed to one of the major ones, lighting. The use of a note book is recommended to remind the aquarist when a lighting change is due, and the notebook is useful for many other memory aids.