Frequently asked questions on sumps


Increasing numbers of fishkeepers are adding sumps to their set-ups. We answer some of your common questions.

How do I ensure my sump doesn’t flood in a power cut?

There are two ways to prevent flooding.

One involves placing the outlet of the pump high up in the main tank, close to the surface. When the power fails the pump will back syphon slightly, but, as soon as the outlet pipe is exposed to the air, the flow will stop.

The second is a popular method if you want the outlet positioned lower down in the main tank.

The easiest way to stop back syphoning is to make the outlet pipe draw in air and break the syphon. If the pipe is low down in the water, drill a 5-6 mm/ 1/4” hole in the pipe near the surface, so again, as soon as the extra hole is exposed, air is drawn in and the syphon broken.

If you don’t want to drill your own holes, use a spray bar return either horizontally, just beneath the water line, or vertically, so that as soon as the first spray bar hole is exposed the same action occurs.

Always ensure there is more than enough empty space in your sump to deal with rapid floods.

What size of sump do I need for my tank?

The bigger the better as it will increase water volume and stability of parameters. But many sumps are dictated by the size of the cabinet.

However, the minimum-sized sump must still be able to hold additional water from pipework and surface area of the main tank when the power goes off. Leaving a gap above all your media and refugium can allow for this extra water. Most ready-made sumps are designed this way.

Ensure that if you illuminate the sump, the lighting is not flooded in such situations.

How can I reduce the noise in my weir?

Too much flow down an outlet or standpipe creates the noisy, sink plughole effect – and the greater the flow the noisier it gets.

Standpipe noise can be sorted at the design stage. Anything more than 2000 lph/ 435 gal down a 25 mm/0.9” diameter standpipe can sound noisy.

If you want even greater flow, as many 300 l/66 gal-plus marine tanks do, use two standpipes, either together or one in each corner behind two weirs.

You can also counteract the bottle 'glugging' effect in standpipes, where water flows down the pipe but air wants to escape up it. As a result the pipe floods slightly before the weight of the water gushes down and the air whooshes back up, making that loud glugging noise.

To counteract this, fit a rigid 'T' piece at the top of the standpipe and ensure it is exposed to the air above the water line. The horizontal part of the 'T' is where the tank water will enter and the vertical part is left free for air to enter. More water can then flow down the pipe, producing less noise.

Fill weirs without standpipes with plastic media to stop splashing noises.   

I’ve been told that my total turnover should be 20 times per hour. Does this water have to circulate through my sump?

In a word, no, and as discussed above, more than a few thousand litres per hour down standard 25 mm/0.9” standpipes can make noise and cause flooding.

When 20 times turnover is recommended, it means by powerheads in the main tank, not by sump pumps. It would be nice to pull water through the sump that often but the reality is that the sump turns into a whirling, splashing, noisy extra tank, unless it is more than 120 cm/48" long.

If you intend to have a refugium in the tank, the manufacturer may well include flow recommendations.

Is there any benefit in using sumps on freshwater systems?

There will always be a place for sumps on very large freshwater tanks, but most tanks – even up to 240 x 60 x 60 cm/94” x 24” x 24” – can be run with a lot less fuss on a couple of large external filters.

Saltwater sumps are more necessary for marine tanks, if not purely to serve as somewhere else to put the equipment. A marine sump can serve a skimmer, calcium reactor, heater, chiller, chemical media, other filter media, automatic top up, water change water, a refugium, a drain, and much more. It can also be used to hold fish and raise young marine fish.

Lessons can be learned from marine sumps and planted refugiums could well be the way forward for future freshwater tanks.

How should I design my sump?

This should be done during early system design to be compatible with your method of filtration.

Will it hold mechanical, biological media and a heater? If so, the standard up and over design with glass baffles will be fine.

Divide the sump into sections with water entering at one end and exiting at the other – but in the middle being forced up and down through the baffles and through media. If adding a sump-based skimmer, leave an extra chamber after the mechanical media.

A deep sand bed or a mud refugium in the sump will need a section, normally at least 30 x 30 cm/12” x 12”, to hold the sand or mud. In these designs the water isn’t pushed down through the sand or mud but flows over it. Water should enter and exit the bed at least 10 cm/4” off the bottom so that the sand/mud won’t get washed away, and because sand beds need to be 10 cm/4” deep to function properly.

If growing Caulerpa over the mud or sand, it must have plastic media either side to prevent it travelling through the sump and clogging the return pipe. The sump also needs to be lit.

Any advice on for how long Caulerpa should be illuminated in a refugium?

Some people light the algae for 10–12 hours each day on a reverse cycle to that in the main display tank. This way the pH swing is counteracted as the algaes in the main tank produce CO2 at night and the algae in the sump uses it up and buffers night time pH values by producing oxygen.

The reverse lighting method makes sense and is used effectively by some, but the algae can still crash as easily as if in the main tank. If you light macro algae 24/7 it is less likely to crash and the population will be more stable and much easier to control.

What weir design is for me?

Make water exit through a hole drilled in the back or base of the tank. Back holes are more prone to flooding and wrongly sited may affect where your water line is in relation to the hood.

Base holes make for easier water level adjustment by adjusting the length of the standpipe, though they normally have a glass or acrylic weir in front. Water floods over the top, draining to the sump. Weirs are always fractionally shorter than the tank rim so water goes over them first.

Do you want water to be drawn from the top of the tank, bottom – or both?

A top-scavenging weir can be simply glued in and water floods over the top, but add an acrylic comb or slots to prevent fish flowing in. This type of weir is really good for clearing scum off the surface.

A bottom-scavenging weir involves fitting an extra plate in front of the glass weir, with slots in the bottom. This breaks the water line, forcing floodwater to exit through slots at the bottom before travelling up and over the normal glass weir behind. Bottom scavenging weirs are good for removing detritus from the bottom of the tank.

A weir can scavenge both top and bottom so fit an acrylic plate in front of the glass one and make slots in the top and bottom. That way you get the best of both worlds. If you are a little confused, specialist tank manufacturers will know what you mean when you mention weirs.