A reef tank is quite different from a freshwater tank or even a saltwater fish only tank. Once a reef tank is set up, it should not be moved unless absolutely neccessary. Because of this it is important to take a little time to consider where to set up your reef tank on a long term basis. Here are some things to consider:
Reef tanks are heavy
Each gallon of saltwater weighs around 8 pounds. Even if a 125 gallon reef tank is filled just with seawater it will weigh around half a ton. On top of the weight of the water, reef tanks have the weight of the tank itself, the weight of the live rock, the weight of the sand and the weight of an additional tank for a sump. It is not beyond reason to have a 6 foot long tank that weighs in excess of a ton! The most important thing about selecting a spot for your reef tank is to choose a spot that is structurally sound. A ton of weight will make 6 inch floor joists fail so if there is any doubt whatsoever, get the advice of a contractor or structural engineer before installing a large reef tank.
Reef tanks are sensitive to changes in temperature
If possible, you want to select a room that remains nearly the same temperature year round. This will elliminate stress placed on the fish by varying temperatures. It is also advisable to keep your reef tank away from doors because of the changes in outdoor temperatures.
Reef tanks are sensitive to light
I have a 46 gallon bow front tank that is tied into the same system as my 125 gallon tank. My 46 gallon seems to be plagued with algae problems while the 125 gallon tank is nearly algae free. The only difference between the tanks is that the 46 gallon tank is near a sun porch. The extra sunlight is the cause of the algae problem.
Reef tanks put out a lot of light
I once set up a frag tank in my office. It seemed like a great idea until I sat down at my computer for the first time and couldn't even read the monitor due to the glare. Consider things like TVs, computers or even areas that you would not like to have excess light before choosing your reef tank location.
Reef tanks can be noisy
If the noise of the protein skimmer or the overflow is going to keep you from sleeping then you will want to avoid your bedroom.
Use common sense
Take a little time and think about the long term commitment of a reef tank before you install it. If you are going to be moving or changing the carpet in two months, you may want to wait until you can set up your tank in a more permanent manner. If you notice a problem shortly after installing your reef tank my advice is to move it before you get livestock in the tank.