Light plays a very important part in the marine aquarium , especially in reef tanks where both fish and invertebrates are kept. Whilst one or two standard fluorescent tubes will provide enough illumination to see what is going on in a 'fishes only' collection, many corals contain beneficial algae within their systems and this requires high levels of light in order to thrive.

Very High Output (VHO) fluroescent tubes are used for lighting reef tanks. Normal sunlight on the reef is at a colour temperature of 5,600^oK but modern marine tubes can provide brilliant white light up to a temperature of at least 10,000^oK.

A combination tube of 50/50 white light and actinic (blue) also provides the correct light spectrum for invertebrate development.

Fitting reflectors to fluorescent tubes will ensure that maxium light is reflected down into the water and not wasted.

Where deep tanks are involved then fluorescent tubes may not be powerful enough and metal halide lamps (often supplemented with fluorescents) may be called for.

These lamps are fitted in pendants and suspended over the open-topped tank - the hood is diopensed with but cover-glasses should be fitted to stop spray damaging the hot lamps and also to prevent fish jumping out.

It is usual to use timers to ensure that the correct length pof tank illumination occurs - a 12 hour period is about right. Using separate timers, it is possible to let the blue actinics come on first followed after about an hour or so by the white lamps; this sequence is reversed at switch-off time and each event respectively simulates dawn and dusk.