Lighting   Wire/ Ceiling Lighting Suspension systems are for Halide ssytems where the unit has to be at least 20cms above the waters surface. Or if you wish a floating design to your tank.  
   

The main reason people use this suspension is it allows units to be suspended 20cm or more above an aquarium. This is mainly to allow heat to dissipate from a Halide Unit before the huge output boils a tank.

This is not the only reason however, some people find it pleasing to allow unrestricted view from above the tank, allowing you to look down on your animals.

The one main disadavntage of this set-up is the loss of water through evaporation. This is aided by your lighting unit heating up the surface. This can be alieviated by placing a condensation tray on the aquarium, however this reduces the effectivness of your lighting.

A ceiling suspension also has to be very secure. You really dont want your expensive lights to come crashing down on your aquarium!

To be sure your system wont move , you need to secure the wire kit. This kit can be bought at most good LFS or they can order it for you. Make sure you get the correct kit for your light.

Wire is used becuase its strong and durable, meaning it wont fray or snap easily. One end attaches to your aquarium while the other goes up into your ceiling. Now this is the tricky bit... you have to wrap the wire around a beam benief your plaster, in a loop knot. trying to screw it in place just wont do.
Please read carefully any manuals carefully when dealing with heavy and expensive lighting units.  
       
Most wire kits have a loop on one end where once you wrap the wire around a beam you put it through the loop BEFORE securing the other end to you aquarium. Please do NOT use plasterboard rawclamps to suspend the wire. A typical Halide unit is in excess of 20KG and that much pressure will rip your plasterboard straight from the ceiling!
To install an overtank system you will need a friend to help stabailse the unit and make sure it level. Please dont try this alone!