Four things to measure
Measure from the water surface in your sump to the return outlet, not from the cabinet floor.
Narrower pipe adds resistance. Match your tank’s bulkhead and the maker’s connection guidance.
Every elbow, valve, tee, reducer, diffuser and narrow outlet costs you flow.
Pick a target for the flow that actually reaches the tank, then find that point on the curve at your measured head.
Worked example: a 1.2m lift
With 1.2m of vertical lift, a pump sold as 4,000 l/h may deliver well under 4,000 l/h. Add a long pipe run, a few elbows and a return nozzle, then read the maker’s curve at that total resistance. If there’s no curve, treat the box figure as the most you could ever get and buy with room to spare.
DC or AC?
DC pumps make it easy to adjust speed, pause for feeding and set a safe fallback. AC pumps are often cheaper and simpler, but usually give you less control. The real question isn’t which is more modern. It’s whether the control, the spare parts and what happens when it fails suit your system.
Bigger isn’t always better
An oversized pump can mean more noise, higher bills, a gurgling overflow and a valve choked half shut. A controllable pump with some headroom is usually easier to tune than a fixed one running flat out, but every extra feature adds cost and one more thing that can fail.
| Check before you buy | Write down | Warning sign |
|---|---|---|
| Real lift | Metres | Using the cabinet height instead of sump water level to outlet. |
| Pump curve | Flow at your lift | Only the zero-head maximum is quoted. |
| Connections | Bulkhead, hose, pipe | Adapters that narrow the outlet for no reason. |
| Upkeep | Impeller access and spares | No clear way to get replacement parts in the UK. |
Read your pump’s flow at your head height →
Follow a worked pump curve example →
THE SHORT ANSWER
Choose from the pump curve, at your real resistance
Two things eat into a return pump’s flow. Static head is the vertical climb from the sump’s running water level to the return outlet. Friction loss comes from the pipe’s width and length, and every elbow, valve, union and fitting. Your pump has to deliver the flow you want after both, so the zero-head figure on the box isn’t the answer.
Pick a sensible target for the flow returning to the tank, estimate your system’s resistance (generously), then check the maker’s curve. Choose a pump that hits your target with room to adjust and a reasonable power draw. Let wavemakers do most of the circulation in the display, rather than forcing extra water through the overflow.
STEP BY STEP
How to do it
- 01
Measure the vertical rise
Measure from the sump’s normal running water level to the highest point of the return. Work in metres, and follow the actual pipe route rather than just the cabinet height.
- 02
Map your plumbing
List the pipe width, total length, and every elbow, tee, valve, nozzle and branch. Narrow fittings and lots of turns cost you flow.
- 03
Set a target for delivered flow
Base it on what your sump needs to process, what your overflow can drain and how much noise you’ll accept. Don’t just pick a big multiple to get more movement in the display.
- 04
Read the curve and leave a margin
Find the pump’s flow at your estimated head, then allow for it slowing as it gets dirty, and for tuning. Check its lowest setting is still usable too.
YOUR SITUATION
What to do in your situation
Low rise, wide pipe and few fittings
A modest pump may hit your target even with a lower headline rating.
Flow drops steeply on the curve
Choose a pump with more head, or simplify the plumbing before buying bigger blind.
The branches need set flows, which change when you move a valve
Plan for the combined demand, or run separate pumps for each job so they’re easier to control.
COMMON MISTAKES
Mistakes to avoid
Treating maximum head as working flow
At maximum head, useful flow is close to zero. It’s a limit, not where you want the pump to run.
Forgetting the outlet
A narrow hose tail or nozzle can wipe out everything you gained with wider pipe further back.
Skipping the power-off test
Your sump has to hold all the water that drains down when the pump stops, and you need to check the siphon breaks in practice.
WORKED EXAMPLE
Choosing a pump for a 1.3-metre rise
A system needs about 800 l/h to reach the tank, through 1.3 metres of vertical lift, two elbows, a valve and a return nozzle. A pump sold as 1,500 l/h at zero head may deliver a lot less against that resistance, so the curve decides whether it’s good enough, not the name on the box.
A controllable pump whose curve shows comfortably more than 800 l/h at that head leaves room for it to get dirty and for fine tuning. Once it’s running, check the final valve and controller setting against overflow noise and sump level, and measure the real flow if you safely can.
BEFORE YOU CHANGE ANYTHING
Write down where you’re starting from
Note the date, the equipment or product you’re using, how you measured and the conditions behind every important number. Say whether a volume is the tank’s label figure or your real net water; whether a flow is the box rating or what actually comes out; and, for test results, the salinity, test kit and time of day. A precise-looking number is useless once you’ve forgotten where it came from.
Change one thing at a time, and decide in advance what improvement will look like: a steady test trend, less detritus, a more stable temperature, a quieter tank or a lower running cost. One reading, or one good day, isn’t a trend, and a setting copied from someone else’s tank isn’t proof it suits yours. The free Reef Companion keeps your tests, maintenance and changes in one dated log, so you can see cause and effect.
MAKING CHANGES SAFELY
Protect your tank while you make the change
Take a baseline
Measure before you install equipment, change your chemistry or alter your routine. Take photos if placement matters, and note how your livestock behaves. Without a starting point, you can’t be sure what caused any difference later.
Start small
Begin with a cautious setting or dose, and keep a way back. Hang on to the old equipment until the new one has proved itself, keep your previous schedule, and don’t add livestock at the same time as a big lighting, flow or chemistry change.
Give it the right amount of time
Mechanical problems show up straight away; nutrient and coral responses can take days or weeks. Judge the change over a sensible period, but stop immediately if you see animals in distress, a leak, overheating or an electrical risk.
Afterwards, note the result and decide whether to keep, adjust or undo the change. If the result is unclear, go back to your last stable setup and measure better, rather than piling on another change.
SIX MONTHS ON
Check it’s still the right decision
What made sense on day one can stop working as livestock grows, the room warms up, equipment gets dirty or a routine turns out to be unrealistic. Set a reminder to review it in six months, comparing what you assumed with the reality: water volume, electricity, consumables, noise, cleaning time and test trends.
Test trends, real flow, temperature, light coverage or whatever the change was for
Is it still doing its job, with room to spare?
How often you clean it, how hard it is to reach, calibration drift and missed tasks
Can you keep this up without cutting corners?
Electricity, reagents, filters, spare parts, warranty and downtime
Would you make the same choice knowing the real cost?
If it’s working, you don’t need to replace it; the review simply gives you a reason to keep it, tune it or plan an upgrade before something fails.
KNOW THE LIMITS
When to get expert help
Get product-specific or professional help for anything involving floor loading, household electrics, unsafe plumbing, damaged glass, unexplained livestock deaths, or chemistry you can’t measure reliably. Use a qualified electrician for fixed wiring, and always follow the current safety instructions for your tank, equipment and additives. Never work around damaged cables, swollen batteries, leaking seals or equipment that keeps tripping the electrics.
Your livestock’s needs can also outweigh general equipment advice. Check adult size, behaviour, disease risk and each species’ light, flow and feeding needs with reputable care guides before you buy. This guide explains the equipment decision; it can’t guarantee the result for every animal or tank.
COMMON QUESTIONS
Frequently asked questions
How do I measure my return pump’s flow?
If it’s safe to do, time how long it takes to fill a container of known volume, or use a suitable flow meter. Keep the setup the same as it normally runs.
Does horizontal pipe add head?
It doesn’t add vertical lift, but extra length and fittings do add friction loss.
Can I turn a DC return pump down?
Usually yes, within the maker’s guidance. Choose a pump that runs efficiently at its normal setting with room to adjust, rather than a huge one turned right down.