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Possible sump layout, showing left to right refugium, return pump, protein skimmer and overflow output from display tank,


At its simplest, a sump is a secondary tank or container that is plumbed to receive water from the main aquarium before it is returned to the main aquarium. Whilst often being made of glass, and with the inclusion of various baffles and compartments, any container that holds water can be used as a sump, such as a plastic tub or bucket.


Most commonly sumps are designed in three sections seperated by baffles (to minimize micro bubble transfer back to the display tank), the first section being where water is overflowed from the display tank and ideally where an internal protein skimmer is situated, the second section utilised as a refugium, DSB (Deep Sand Bed) and facility to grow Macro Algae (that takes up nutrients and can then be exported/removed by trimming/pruning back)this section needs to be under light(s) to promote macro algae growth, the third and final section is for the return pump - it is also the section that will indicate the optimum water level of the system when running. This level needs to be set at a height that will allow water returning to the sump when all power is switched off or in the event of a power outage but not so low as to run the return pump dry. It is beneficial to mark this level on the side of the sump with a permanent marker, tape or Dymo label to indicate how much water is lost through evaporation. Water evaporation could be in the vicinity of 5 to 10 litres per day on a 4x2x2 aquarium with two 250 watt Metal Halide Lamps. Note: water that has evaporated from a saltwater aquarium is topped up with fresh water (not saltwater) as only the water has evaporated not the salt content.

Hobbyists numbers today are continually growing and so to are the size of their aquariums, in the past a 6x2x2.5 aquarium would have been considered large, a quick browse through the RTAW Tank Journal forum and one will see the influx of home aquariums 7', 8' and 10' Wide by 3' and 4' Deep and up to 3' High. Consequently, the size and style of sumps has also advanced and we hear terms like "RaceTrack design", effectively this means that the sump is partitioned lengthways as well and water flows down one side and back the other and incorporates more sections for various other components/options, perhaps a dedicated area for filtration media, a DSB (in addition to the refugium), a compartment to house Zeovit, calcium reactors or fluid bed filters, a coral frag facility or even an isolated compartment to be used as a fresh water top off reservoir.


The major benefits of having a sump are:

  • It increases system water volume, for improved stability of water parameters;
  • It allows for the maintenance of constant water level in the display tank, with water level variations due to evaporative losses occurring in the sump.
  • It provides a place to put junk that would be preferable to not have in the tank, such as:



The most common configuration is to have water extracted from the main tank by means of a weir into an overflow area and fed by gravity through a drilled hole in the base of the overflow area to a sump located under the tank. A pump is then positioned in the sump to return the water to the main tank. This system will self equalise, such that when the pump is running, water will be fed to the sump at exactly the same flow rate at which it is returned to the main aquarium.

A benefit of this configuration is surface skimming. Utilising an overflow weir means that water is constantly being withdrawn from the surface of the display tank as it is drawn over the overflow. This prevents the build up of scum on the water surface. Additionally, the air/water surface is larger providing for more gas exchange.


Other alternatives for extracting water from the main tank include use of a standpipe (with various configurations such as 'Durso', or 'Stockman'), a bulkhead in the side of the main tank, overflow boxes, or overflow siphon plumbing. These all have various advantages and disadvantages, but most are considered as second-best options to try and make use of a tank that hasn't been drilled and had a weir installed.

Whenever possible, choose to install a weir before the tank is filled with water. If not, chances are that it will regretted. TRUST ME!

Avoiding Wet Floor

It is worth noting that when the return pump is turned off, it will form a siphon and water will backflow through the return line, through the pump, and back into the sump. This will continue until water drops to such a level that air is drawn into the return line and the siphon is broken.

Care must be taken to ensure that the capacity of the sump is such that it will cope with the volume of water that backflows until the siphon is broken. For this reason, sump return pipes are normally placed near the top of the display tank to minimize the amount of backflow before the siphon is broken, and so that a backflow siphon does not totally empty the display tank in the event of a power outage. Alternatively, a series of small siphon break holes may be drilled into the side walls of the return pipe just below the water surface level in the display tank to enable air to be drawn into the pipe as soon as the water level drops by a small amount.

Setting Sump Water Level

  1. punch a 3mm hole in your return line about 10-20mm higher than your weir
  2. fill display tank with water to top of weir so it just starts to trickle over.
  3. fill weir to level it will drain to when pump cuts out i.e. height of lower of standpipe or siphon
  4. fill sump with water to within 50mm of top rim
  5. mark outside of glass at this 50mm level with a waterproof pen
  6. switch on return pump
  7. wait 2 minutes
  8. mark this sump 'running' level with a waterproof pen
  9. that's it, you are done.

If super cautious (aka paranoid), use the following steps instead:

  1. punch a 3mm hole in your return line about 10-20mm higher than your weir
  2. fill display tank with water to top of weir so it just starts to trickle over
  3. fill weir to level it will drain to when return pump cuts out i.e. height of lower of standpipe or siphon
  4. fill sump with water to within 120mm of top rim
  5. switch on return pump
  6. wait 2 minutes
  7. switch off return pump
  8. wait 30 minutes
  9. top off sump with water to within 50mm off top rim
  10. mark this 50mm high water level with a waterproof pen
  11. switch on return pump
  12. wait 2 minutes
  13. mark outside of glass at this 'running' level with a waterproof pen

Required Hole Sizes

Here are the fitting size followed by the approximate minimum hole sizes required for typical bulkhead fittings:

  • 25mm = 36mm
  • 32mm = 45mm
  • 38mm = 51mm
  • 50mm = 63mm