Many aquarists have a sump attached to their display aquarium. The numbers that have a sump seems to advise that the answer to the title question is ‘Don’t go without a sump’.
In fact there isn’t a requirement to have a sump. There are a good number of marine systems that do not have one and they are successful. This applies to both fish only and reef aquariums. There are articles on aquaristsonline.com that suggest a basic kit list and a sump is not included. So why then do so many aquarists use them?
A sump is another seawater container that is often placed beneath the display aquarium though it could be elsewhere provided that gravity is available to feed seawater to it. The sump is more often than not another and usually smaller aquarium. Normally an overflow in the display aquarium is connected to pipe work that carries the seawater down, and the seawater is lifted back up to the aquarium by a pump. The guideline for the speed seawater moves through the sump is the system net gallonage times three.
The size of a sump is simple to calculate, it should be as large as is practical. The larger it is means that it will contain more gallons and that is the first advantage. With a sump the gallonage of the system is increased, which is good for seawater quality. The gallons held in the sump should not be used in any stocking calculation as this would negate the quality aspect of the extra gallonage and, with fish, there is the danger that the physical area of the display aquarium would be overloaded resulting in disputes and aggression.
Heaters and a protein skimmer intake are not exactly natural looking in the display aquarium, though many aquarists manage to reasonably hide them. If a sump is available the heaters can be placed in there and a skimmer can stand alongside, in or hang on. This removes the ‘eyesores’ from the display area and probably makes them more accessible. The protein skimmer in particular needs to be easily accessible for regular maintenance.
The usefulness of a sump doesn’t stop there. Seawater filtration is very important and the move nowadays is towards natural methods. Live rock is a good example. Using a deep sand bed (DSB) or plenum (a raised sand bed) is a good partner for live rock offering good additional filtration, and the best place to locate such a sand bed is in the sump. The guideline for the area of a DSB is 2/3rds of the display aquarium base area. This isn’t always achievable so the largest sump that will fit is again an advantage. The DSB is deep, hence the name, but this doesn’t stop the heaters and protein skimmer intake from going in the sump as well.
Still on the filtration theme, some aquarists grow the macro algae Caulerpa in the sump, which is designed as an aid to filtration and seawater quality. The Caulerpa will help remove, among other things, nitrate and phosphate. These two are implicated in the growth of nuisance algae.
The sump should not be used as a refuge for a sick fish as it forms part of the overall system seawater gallonage. For a fish suffering from excessive harassment it could be a temporary home. Also, if a life form appears that is unwanted the sump could again be used as a temporary home. A Mantis shrimp for example.
There isn’t a requirement to have a sump immediately the system is set-up, although this is really the better way to go – have the advantages from the start and avoid moving equipment later. A sump could be added later and in this case it is advantageous to consider it at the design stage. Though there are siphon based overflow boxes available, the best way of getting seawater to the sump is into overflows and down pipes. This requires a hole or holes in the display aquarium. These should be drilled before the system is set up. If they are not going to be used straightaway they can be temporarily covered.