An aquarium sump should contain enough water to allow your sump-based filtration equipment to function properly and to account for evaporation, with enough room left over to hold water that flows into the sump from the aquarium in the event of a power failure. As is often the case with an aquarium, bigger tends to better -- a larger volume of water provides more stability and is overall more forgiving of equipment malfunctions and human error.
Siting an Aquarium Sump
When you're designing a new aquarium setup or just adding a sump to an existing setup, you will usually want to use the largest size of sump that will fit under your aquarium stand. If your sump is located in a remote location, such as a separate fish room adjacent to the room where the aquarium is located, you may be able to fit a sump that's quite large. Some serious aquarists have sumps that are hundreds of gallons. These may service multiple tanks and can seem more akin to lagoons than to sumps.
Many aquarists prefer sump-based filtration equipment over in-tank filtration equipment for aesthetic reasons. A sump-based protein skimmer and mechanical filter can save space inside an aquarium stand; some media reactors for chemical filtration can hang on the side of a sump. Most of these sump-based filtration components require small power heads or submerged pumps to supply them with system water -- the sump must hold enough water to keep these power heads under water at all times.
Maintaining Water Level
One of the advantages of using a sump is that the water level in the display tank will remain constant. This does not mean that water is not evaporating from the system, but the water level will drop only in the sump. Because of this, it is important to regularly top off the water in the sump either manually or with an automatic top-off device. In the event of a power failure, some water will drain from the display tank back into the sump, so you must ensure the sump basin has enough capacity to handle the back-flowing water without flooding over the sides.
Sump Water Level
In most cases, the sump will be only three-quarters full, unlike the display aquarium, which will be full to within an inch or 2 of the top at all times. Many factors will determine exactly how full your sump is. Make sure all power heads servicing sump-based filtration equipment are submerged at all times and that plenty of room remains to accommodate the back-flow from the display tank during a power outage or return pump failure. As a general rule of thumb, the more water in the system the better, so long as you take precautions to avoid flooding over the top.
Other Sump Size Considerations
Many aquarists like to have a sump big enough to house various probes, heaters and other equipment. In addition, some aquarists choose to have a refugium built-in as an integral part of a sump. In the case of very large remote sumps, some opportunities exist for keeping additional animals.