There is always an element of danger when keeping water and electrical devices together in such a confined space. Problems can and do occur at some point and it is better to be weary about what could happen than not. Below are a few points that you may or may not already know about keeping electrical devices in the aquarium.
Heaters expel a tremendous amount of energy below the water so they can heat an aquarium efficiently. When keeping your heater in the aquarium, make sure it is submerged low enough in the tank to avoid being exposed to air, especially when doing a water change. When siphoning water into a bucket it is easy to monitor how much water is coming out at any one time. Not so if you have a larger tank and are siphoning it directly into a drain. It is easy to walk away, become distracted and come back to find the water line lower than it is meant to be.
Even a heater that is partially exposed to the air, in a very short period of time can explode once it gets hot enough. This explosion is enough to shatter the glass of your heater and expose the internal elements to the water. This should cause the circuit breaker in your home electricity box to trip, cutting power to your aquarium heater. To help increase safety, use a power board with a safety cut out switch to avoid any potential problems that could occur. Or better still turn off the heater while doing a water change.
If your filter leaks, or any of the hoses attached to your filter become dislodged, water is going to drain out of the aquarium down to the lowest point where it is siphoning from. Your heater should be below this point so it is not exposed to the air if this does occur. If this happens, your filter may overload and cut out, possibly tripping a circuit breaker. The same holds true for any powerheads. If a powerhead become exposed for any reason they will become increasing hot until they seize up. The impeller and motor below have melted together from over heating.
This is ok if it happens in front of you as you can attend to the problems that arise. But if this was to happen when you are not home or have gone away for the weekend, your fish may suffer from no filtration or aeration from a tripped breaker. The dissolved oxygen levels in your tank will start to decrease, thus suffocating your fish. In a very short period of time this will also kill off the beneficial bacteria in the filter that helps maintain a harmonious balance in the aquarium. Once this bacteria is dead, the nitrogen cycle will have to be started all over again.
If you can, keep your filter and heater plugged into an outlet that is on one circuit breaker and your power head and air pump on the other circuit breaker. If one, or the other cuts out for any reason then you at least have filtration or aeration on your tank at all times. The main idea is to keep constant movement in your tank. If you live in an area that is susceptible to a lot of black outs then it is advisable to have a battery back up for your air pump in case of emergencies.
Making your electrical equipment secure is as easy as cable tying or clipping it to the frame the tank sits on or any other solid object present. This will stop any cord from accidentally being pulled if something falls onto it. To avoid any future problems that may arise, secure all cables and hosing. Always loop the cord of any electrical devices down lower than the electrical outlet. If water does run down the cord for what ever reason, it will not run directly into the outlet, cutting off the power.
You have heard this thousands of times, water and electricity do not mix. Some aquarium owners may never run into any of the problems above. It only has to happen once to become a major headache. It is better to be prepared and help avoid any potential danger that could occur, than to be cleaning up any mess that is detrimental to the health and well being of your fish.