What Is a Sump, And What Does a Sump Have to Do with a Home Aquarium?
(The First Tank Guide)
So, what is a sump? A sump is simply a container to collect water from a plumbed system. So, what does that mean? It means that if you have tubes and pipes (plumbing) coming out of your tank to carry water to filters or other equipment, you may need some place for water to collect. This place for water to collect is your sump.
Closed or Open Systems
One thing that is important in determining whether you need (or even want) to get a sump for your home aquarium is whether your fish tank is an open or closed system.
Most aquariums are closed systems. In a closed system, there is no place for water to leak out. Canister filters, ultra-violet sterilizers, and fluidized bed filters are all closed systems. Water is taken out of the fish tank through a tube, pumped through the sterilizer or filter, and returned to a point inside the tank. This is called a closed system because there isn't any point along the water's path where it is not controlled and contained. Closed systems do not need a sump.
Open systems have some means within the system where water could - or is even expected to - escape. Trickle filters and protein skimmers are open systems. These pieces of equipment are designed with openings to allow the release of water, air, or waste. Because these systems are open, they need something to catch the water that can (or does) pass through. Though they can be set up so that this can fall back into the tank, usually it is the sump that catches this overflow.
Aquarium Water Return
Water is returned from the sump to the main tank. This is accomplished with a circulation pump placed in the sump (if the pump is submersible) or plumbed in adjacent to the sump (if the pump is not submersible).
In most cases, water is pumped from the sump into the aquarium, and then allowed to flow back into the sump. In this way, the pump is allowed to run continuously. In some cases, however, there may not normally be water getting into the sump. In these cases, the pump needs to be switched on and off as water collects in the sump. This is normally accomplished with a float switch, similar to the floats that turned off the water in old-style toilets, or the switches that automatically turn wells back on when stock water tanks start to get low.
Since the main reason for having a sump set up with your aquarium is to keep water (or aquarium waste) from spilling out onto the floor, you need to make sure that your sump is large enough to do that in the case of a problem.
In general, this means that your sump needs to hold all the water that can drain out of the plumbing, filters, or anything else in the case of a power failure. That doesn't mean that the sump needs to hold all the water in the tank too, the tank should not be able to drain in the case of a power failure. The sump just needs to hold all the water in pipes, plumbing, tubes, or other containers that allow water to flow into the sump in the first place.
Usually the easiest way to do this is to set things up and get things running, then turn off the power to the tank and let water drain into the sump. This will tell you how much water drains out of everything into the sump. While the power is still off, fill the sump close to the top. Then turn the power back on. As water is pumped out of the sump and back into all the systems, you will see the water level in the sump go down. After everything has been running for a few minutes, maybe half an hour, you will see the level the water is lowered to in the sump. This is the HIGHEST level the water can ever be in the sump and not risk a problem in case of a power failure. If the sump is the right size for the system you are running, you should have plenty of water still over the pump to allow for safe operation. (You will probably want to mark this level somehow so you know how much water the system can take.)
Remember, in a system with a sump, the overflow in the tank will keep the water level in the tank constant. Any water that is evaporating from the system will be missing from the sump, and if you are not paying attention to this level you can expose the pump and risk overheating or other problems. Also remember that if you add water to the system, you will probably not see any meaningful rise in the water level in the tank - the rise in water level will be in the sump. Be sure to never fill it above the maximum level you determined above.
Uses of a Sump
In addition to their primary purpose collecting water (or possibly waste) and preventing a spill or damage, sumps can also be used to house unsightly equipment, or equipment that needs to be out of the tank for some reason.
The most common use of a sump is in conjunction with a trickle filter. Generally trickle filters are set up to allow water to flow through the "tower" of the filter and drop into a sump. Water is then pumped back into the tank to repeat the process.
Sumps are often used to house protein skimmers, though many skimmers can be set up in the aquarium. The sump provides a location out of the fish tank for the skimmer, which can be both large and unsightly. In fact, the higher quality skimmers really can't be set up in the tank. By setting the protein skimmer up in the sump rather than out on its own, the sump then can act as a back-up in case you forget to check the catch as often as you need to.
Once people have a sump connected to their aquarium, the aquarium heater is often one of the first things moved to the sump. This can also be important if the aquarium houses particular aggressive of belligerent fish that may damage the heater. However, you need to be aware that if the heater is in the sump, it is only indirectly heating the tank, so it is even more important to monitor the temperature in the sump and in the tank.