Sumps and large systems

A sump is a second tank underneath the display with the filtration and the equipment inside it. Water leaves the display over an overflow, falls into the sump, passes through the media, and is pumped back up.

The reason to fit one is not what most people think, and the sum that decides whether it is safe is one almost nobody does before buying the parts.

What a sump is actually for

It is not better filtration. Biological filtration is a function of media surface area, oxygen and flow, and a good canister filter delivers all three. Nothing about putting the media in an open tank makes the bacteria work harder.

What a sump gives you is two things, and both matter to a goldfish keeper:

Water volume. Dilution does much of the work in water quality, and this site’s whole argument about tank size is that goldfish need volume rather than filtration to stay in clean water. A 70-litre sump under a 300-litre display is 23% more water for no extra floor space, and every parameter moves more slowly in it.

Somewhere to put things. Heater, thermometer probe, media, dosing, an auto-top-off float — all out of the display. That is aesthetic for most keepers and genuinely practical for anyone keeping telescopes or bubble eyes, whose eyes are damaged by exactly the hard-edged equipment a sump removes.

Everything else — media capacity, servicing without disturbing the fish, a display water level that never drops — is real but secondary.

How the water actually moves

Worth having clearly in mind, because every failure mode follows from it.

1. The display is drilled, or fitted with a hang-on overflow box. Either way, water leaves at a fixed height — the weir.
2. It falls down a drain pipe into the sump, entering the first chamber.
3. It passes through mechanical media first — a filter sock or coarse sponge catching solids — then biological media, which is where the nitrogen cycle happens. The filter media guide covers what goes where and in what order; a sump changes the packaging, not the principle.
4. It arrives in the return chamber and is pumped back up to the display.

The system is a loop with one pump in it. When the pump runs, the display level is held at the weir and cannot change. When the pump stops, gravity keeps working and the loop becomes a one-way drain.

One consequence is pleasant and often missed: evaporation shows up in the sump, not in the display. The display sits at the weir permanently, which looks better and keeps the surface agitation constant. The corollary is that the top-up job moves to the sump — and because the sump is a fixed vessel with a visible level, it is the one place in a goldfish system where an automatic top-off is genuinely straightforward.

The drain-down calculation

This is the part that decides whether a sump is a piece of equipment or a flood, and it is not difficult. It is simply skipped.

When the return pump stops — power cut, tripped socket, failed impeller, unplugged for maintenance and forgotten — water drains backwards from two places at once:

A. Everything in the display above the weir. The display drains down until its surface reaches the top of the overflow teeth. That is the tank’s surface area multiplied by the depth from the running water line down to the weir.

B. Whatever the return line holds. The return pipe and nozzle siphon backwards down to the level of the return outlet, dragging display water with them until air breaks the siphon.

Both arrive in the sump. So:

The sump must have at least A + B of empty space above its normal running level. Not its capacity — its free space while the system is running.

A worked feel for the numbers: a 120 × 45 cm display with its water line 2 cm above the weir teeth holds 10.8 litres in section A alone. Drop the weir another two centimetres and it is nearly 22. It adds up far faster than people expect, which is why “the sump looked big enough” is such a common description of a wet floor.

Kill the return siphon rather than calculating it

Section B can be designed away almost entirely, and should be.

Drill a small hole in the return pipe, just below the water surface, pointing into the tank. When the pump stops, that hole immediately draws air and breaks the siphon. Section B becomes a few hundred millilitres instead of several litres.

It is crude. It has nothing to fail. That is the point.

Then test it, once, deliberately

The calculation tells you what should happen. The test tells you what does.

With the system full and running normally, switch the return pump off at the wall and watch. Let it settle completely. Mark where the sump level ends up. If it is close to the rim, the sump is too small or the weir is too high, and you have found that out on a Tuesday afternoon with a towel to hand rather than at three in the morning.

Sizing the pump, and the figure on the box

The return pump’s rated flow is measured at zero head — pumping into free air with no pipe on it. A sump under a tank asks it to lift water a metre or more, through elbows and a valve.

So read the head curve, not the headline number. How much a pump loses depends on its maximum head and on the lift and bends it faces; the curve shows the flow at the height you actually need, and the difference is invisible until the system is built.

Two constraints then apply at once:

• The turnover target is the same one the filtration guide sets for any goldfish system, applied to the whole volume. Use the stocking checker to sanity-check it against the fish you actually keep.
• The overflow’s capacity is the hard ceiling. It must carry everything the pump sends, quietly, with the drain partly fouled. In practice this, rather than the pump, is what limits a home sump — and it is why over-pumping a system is a mistake rather than insurance.

The fall into the sump does one thing for free that is worth naming: it agitates and gasses the water thoroughly, which is aeration without an air pump. On a heavily stocked goldfish system that is a real contribution.

Running one: what actually changes week to week

Less than people expect, and one thing more than they expect.

The mechanical stage is the only frequent job. A filter sock or coarse sponge in the first chamber catches solids and clogs steadily. Rinse or swap it weekly, or more often on a heavily stocked system. A clogged sock does not stop the system — it overflows its own chamber inside the sump, which is harmless but means the solids bypass the mechanical stage entirely.

The biological media is left alone, on the same principle as any filter: disturb it as little as possible, and rinse it in tank water rather than tap water when you must. The cleaning guide covers why.

Water changes get easier. Siphon from the display, where the waste is, and refill into the sump, where new water mixes and comes up to temperature before it reaches the fish. That second half is a real improvement on pouring water into a tank containing goldfish, and it is one of the quieter arguments for a sump. The water change guide is otherwise unchanged.

And the thing that catches people out: dose to the whole system. Salt, medication and conditioner all work on the total water volume, which now includes the sump, the pipework and everything the system holds — not the number on the display tank’s box. Dosing the display volume on a system with a 70-litre sump under a 300-litre tank delivers about four-fifths of the intended concentration, which is exactly the way treatments fail quietly and resistance develops. Using medications safely makes dosing by real volume its central point, and a sump is a common reason the real volume is not the obvious one.

What it costs you

The whole list, because the benefits are the half that gets advertised.

A permanent flood risk. Managed, calculated and tested, but never zero. A canister is a sealed loop; a sump is an open system with gravity in it.

A pump that must never stop, and will. The power cuts page covers running a tank through an outage, and a sump raises the stakes: the drain-down happens in the first minute, and what it does to the biological media depends on the design. Media above the water sits in air, so the risk is drying out; media that stays submerged in a stalled sump is the oxygen-poor case.

Noise. The overflow is often the noisiest part of a home aquarium.

Evaporation and humidity. More surface and more agitation mean significantly more water into the room — which the placement guide treats seriously, because it is a problem for the wall rather than for the fish.

Weight, on one footprint. This is the cost most likely to be genuinely serious. A 300-litre display plus an 80-litre sump plus glass, stand, substrate and cabinet is comfortably over half a tonne in one place. Read the floor loading section before buying anything, particularly above ground level.

Money. Drilling or an overflow box, the sump, baffles, the pump, pipework, unions and valves. What keeping goldfish costs sets the general shape; a sump is squarely in the equipment third of it.

Central systems: one sump, several tanks

The natural next step, and the one with a consequence the site has already stated elsewhere.

The attraction is obvious: one filter, one heater, one water change, one set of tests, and a combined volume that dwarfs anything a single tank could hold. For a breeder running grow-out tanks, or anyone with a wall of display tanks, it is a large saving in labour.

The cost is that every tank becomes one epidemiological unit. Shared water means shared pathogens: a fluke, a protozoan or a bacterial infection in any tank is in all of them within hours. This is precisely the mechanism the choosing guide warns about in shops, where a bank of tanks on one sump means the fish you are looking at has been swimming in water from every other tank on the run.

Two things follow, and they are not optional:

Quarantine cannot happen inside the system. A quarantine tank or a hospital tank plumbed into the same sump is not a quarantine tank. It needs its own water, its own filter and its own heater, standing completely apart.

Equipment is part of the shared water. Nets, buckets, siphons and hands move pathogens between systems as effectively as plumbing does. A separate net and bucket for the quarantine tank is the whole of the discipline, and it is the part people skip.

A central system is therefore a reasonable choice for a display collection you are willing to treat as a single animal, and a poor one for anybody bringing new fish in regularly.

Indoor ponds, tubs and stock tanks

Worth knowing about, because it is where a lot of serious goldfish keeping actually ends up, and it sidesteps most of this page.

A large plastic stock tank or a purpose-built indoor pond gives you the volume of a very large aquarium at a fraction of the price of the equivalent glass, and it is filtered the way a pond is: an external pump feeding a gravity or pressurised filter box, hose-connected, sealed, with no drilled glass and no drain-down problem to solve.

You lose the side view. For ranchu and the other hooded breeds that is not really a loss — they are a top-view tradition, which is why the wen care and showing pages both keep mentioning tubs. For anybody who wants an aquarium in a living room it is decisive.

The alternatives, in the order worth considering them

Before committing to plumbing, three of these get most of the benefit:

1. A bigger display tank. Always first. Volume with no pump, no plumbing and no new failure mode. If the reason for the sump is water volume — and it usually is — this answers it directly. The choosing a tank guide covers the trade-offs.
2. An external pond filter on a tank. A pressurised or gravity box filter driven by an external pump gives you unlimited media and easy servicing in a sealed hose loop. It is a canister with more room, and it carries none of the drain-down arithmetic.
3. Two canisters rather than one. Redundancy, staggered servicing so the bacteria are never all disturbed at once, and flow from two ends of the tank. The buying guide covers what to look for.
4. A sump, when the display is large, the volume is worth having, the floor can take it and the plumbing does not worry you.

When a sump is the right answer

Concretely, it is worth it when several of these are true at once:

• The display is over roughly 400 litres (a rule of thumb), where the sump’s volume is a meaningful addition rather than a rounding error.
• The tank is already drilled, which removes the most awkward part.
• You keep fish whose eyes or fins are damaged by in-tank equipment.
• You are running an indoor pond or a genuinely large system.
• The floor, the room and the noise tolerance all say yes.
• You will do the drain-down sum and then test it.

And it is the wrong answer in a rented home, upstairs on a timber floor without checking, on a first goldfish tank, or for anyone who has been told it filters better. It does not. It holds more water — which, for a goldfish, is very nearly the same thing as being better, but it is worth knowing which one you are buying.