There are many aspects in the aquarium which generate heat. There are pumps, heaters and of course the lighting.
The choice of lighting normally determines if you will actually use a hood or not. If you choose to use fluorescent tubes then quite likely you will implement a hood or canopy as they are sometimes known. An aquarium hood may be used with metal halide lighting, however this will probably be determined by the choice of metal halide. If you elect for a pendant type then you may not use a hood, however if you use another type of metal halide then you may choose to use one.
As we all know heat rises and if a hood is chosen then all the heat which is generated by the various aquarium equipment will build up in the hood and therefore could have a detrimental effect on the water temperature. Something we dont want to happen is an increase (or decrease) in water temperature. If the heat builds up in the hood then you will also experience a higher level of water evaporation which could mean fluctuations in salinity plus you could experience an electrical failure due to the elevated heat levels.
The good thing is that it is really easy to rectify.
What you can do (and this is one solution out of many) is install fans into the aquarium hood.
Small fans will suffice and could be installed in both sides of the hood. The fans on the left could be installed to blow cool air from the room into the hood and the fans at the other end installed the other way around so that they are blowing air out of the hood. This way you will get an ever moving flow of cool air through the hood.
There are some people who drill holes in the hood rear or top. Whilst this does remove some of the heat I personally do not believe that this will allow for the heat inside the hood to cool down quick enough – unless the holes are of a large size. I personally believe that there needs to be cool air drawn into the hood.
The ambient temperature of the room also needs to be taken into consideration. If it is a hot day for example then the air surrounding the aquarium will be warm and unless you have invested in an aquarium chiller then probably you are going to struggle to maintain the heat within the aquarium. In these instances if you do not have a chiller it is probably best to remove the top from the hood to allow some of the hot air to escape.
Not always possible though is it as you are not always at home.
Computer fans in my opinion are a good option to use as they are cheap to purchase and easy to install, however there are many other fans which can be used. What you need to look at is how you are going to power them. I suppose an interesting way to do this would be via the use of solar power. This way you could use the lighting as the source of power. When the lights are on the fans are activated and when the lights turn off the fans turn off. Perhaps solar powered fans from a greenhouse could work.
Of course another way would be to just hook them up to the power outlet!