The Best Accessory – A Sump

There it is, a beautiful fish only or reef aquarium. All is done, a variety of fish swimming about, if a reef among and over colourful corals. What can assist the life in the aquarium and make it more interesting for the aquarist? If fully stocked it shouldn’t be another fish or coral.

So, nothing to be done then. Well there is something that is of great benefit and expands interest even further. It is suitable for a fish only or reef system. It can be added later or when the aquarium is being put together. It’s a sump.

A sump is definitely not an absolute must by any means. There are plenty of successful marine aquariums about that don’t have one. However, there are benefits to installing a sump.

If the aquarist is good at basic silicone and glass construction, the sump container and divisions can be completed by him/her at low cost. If there is unsurety about keeping things watertight, then obtain a ready built aquarium and put divisions in, if the silicone used for the divisions is not particularly neat and professional no matter as it’s all in a cupboard. (Note that silicone suitable for aquarium use must be used.) Or get it all built professionally.

Usually, but not necessarily, a sump is installed in the cupboard below the aquarium. This puts it out of sight and at the same time gives easy accessibility. It is simply a glass box which very obviously needs to be watertight.

So the first requirement is to decide what size the sump is to be. It needs to be the maximum size possible at the same time leaving room for plugs and sockets, any power filter (which is possibly going to be redundant anyway) plus any other aquarium paraphernalia that can’t be housed elsewhere. In addition it needs to be remembered that access is required, from the two sides to an extent but mainly from the top, the aquarist needs to be able to reach everywhere inside reasonably comfortably. Also check the height of any equipment that might go into the sump and protrude above the seawater surface such as a protein skimmer. Be sure that the sump, front to back, is large enough to take the heater(s), these are usually very easily installed.

The simplest but still effective glass divisions within a sump are two giving three spaces. The first division leaves a narrow first space to take heater(s), the second division leaves the widest empty space and the final division gives enough space for the return pump and protein skimmer. It follows that the size of the return pump and protein skimmer need to be known before the spacing is decided. Once that is done, the sump construction can proceed. The seawater needs to flow between all three spaces without any unwanted restriction. This is achieved by having overflows (underflows as well on more complicated sumps). So the overflow at the first division must be high enough to cover the heaters, easily done. The first division should be high enough to at least equal the height of the second division and the second division height is determined by the requirement of the return pump and the protein skimmer leaving a fair amount of seawater above them. In effect, the first division could be the same height as the second.

So, the seawater comes in via the heater area, overflows the first division, fills the middle section, overlows the second division and is pumped back to the aquarium at the same time as being skimmed.

What of the middle section then, what’s that for? Let’s wait until the seawater coming from and back to the display aquarium has been considered.

The return pump which sits in the sump has been mentioned. Suitable flexible piping takes the seawater to the aquarium over the top rim at the right of the aquarium (not an absolute but the pump is on the right in the sump). Gravity is used to take the seawater down into the first section of the sump. The best way to achieve this is by having a drilled aquarium. This drilling is easier than might be thought, but sensibly any doubt and a professional can do it, either before the aquarium is taken home or when at home. The seawater will flow through a curved right-angled fitting which is placed and sealed in the drilled hole. Then flexible tubing is connected to the fitting and a secure outlet placed in the sump, the first section. To prevent any mini-bubbles a pure white small cotton bag can be used over the outlet making sure the water passes through the bag without restriction, though this could be found to be unnecessary. It is necessary to place the drilled hole as high as possible to ensure that the top water-line in the display aquarium is acceptable. This doesn’t present a problem though, as a curved right-angle corner can be placed inside the aquarium on the fitting which can be swivelled to achieve the seawater level. For security and peace of mind it is best to use a fitting and overflow pipe larger, even twice the diameter, as the pumped inlet pipe. As a check, when the system is operating turn off the pump and ensure the seawater doesn’t rise excessively (ie overflow) in the sump as some seawater will still go down.

‘Er, wait a minute, my aquarium is full of seawater and fully stocked.’ No problem, reduce the level of the seawater for a while, giving time for the hole to be drilled and the sealing silicone to set (24 hours is often quoted for the silicone). There is usually enough seawater headroom in the aquarium so level reduction is not a problem for the aquarist or livestock.

Another way of getting seawater to the sump is without drilling. This uses a siphon, a basic one is constructed of a curved piece of pipe over the edge of the aquarium and a tube going to the sump. As the pump sends seawater up so seawater flows down the siphon. There could be problems, the first is if the pump stops as the siphon will continue to operate before the seawater level breaks the siphon. This is easily checked by allowing seawater to flow down with the pump turned off – when the seawater in the sump has risen to fairly near overflowing mark the siphon and shorten it. This is then the point at which the siphon will break. Flow rates are important with a siphon as well. I would say a drilled system is the one to use, siphons don’t make me happy, a very small amount of extra effort is worth it. Maybe that’s just personal. There are more sophisticated siphon systems.

The sump can have a basic cover over it such as plastic. It will help control humidity in the cupboard it’s in etc. It will possibly need cutting so it will go over any protrusions such as the protein skimmer.

Ok, so what can be placed in the middle section of the sump then? First of all, nothing! It can be left empty for as long as wanted. Perhaps later interest will suggest say a deep sand bed (DSB). This will assist the filtration of the aquarium and give additional interest for the aquarist, with loads of minute ‘beasties’ going about. Perhaps, if nitrate is a problem, seaweed such as Caulerpa can be grown, this will use nitrate and when harvested some nitrate will go with it. This means lighting is required though, on at the opposite time to the aquarium lighting. But these things can come later.

The basic sump as described is easily obtained and fitted, with perhaps a little professional drilling. As said it brings advantages: it moves much of the technical equipment out of the main aquarium thus enhancing the display, it increases the seawater volume making the ‘seawater per fish’ greater and also gives the aquarist the opportunity to widen the marine horizons by developing perhaps a DSB etc. Are there any disadvantages? Not really though the seawater amount changed routinely will increase a little. The guideline, at least at the start, is 10% of the net seawater gallonage including the sump so the sumps net gallonage needs to be known. Also additional fish should not be introduced to the main aquarium because of the additional gallonage in the sump, the fish are still in the confines of the aquarium and it could cause stress. The advantage of the additional seawater will also be lost of course.

The sump described here is basic (nothing wrong with that). Using a search engine and inputting ‘marine aquarium sump’ or similar will bring up a lot of information, ready built units for sale, kits for sale and different designs. A sump is definitely worth having giving the aquarist more scope and the aquarium inmates a bit more of the ‘ocean’.

Finally, often in the below aquarium cupboard are electrical outlets, sockets and similar. Salt water and electricity are potentially dangerous. With sensible care no problems will arise.


  1. Great Article! Really enjoyable read!

  2. Glad! If you haven’t got a sump maybe you’ll be tempted.

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