The two calculations
The flow your return pump actually delivers ÷ your net water volume. It tells you how often water passes between the display and the sump.
Wavemakers move water around inside the display. How well that works depends on direction, spread, rockwork and how your corals respond.
A measured 1,200 l/h from the return on a system with 200 litres of real water is 6× turnover.
Don’t add every pump’s maximum rating together and call it your “tank flow”. Return pumps and wavemakers do different jobs.
A worked example
Say your tank is sold as 250 litres, but rock, sand and the sump’s water level leave about 210 litres of real water. If your return delivers 1,050 l/h at the outlet, your turnover is 1,050 ÷ 210 = 5× an hour. That tells you far more than the 3,000 l/h printed on the box, which is measured with no lift at all.
How to find your real return flow
Best: measure it at the return outlet with a container and a timer, or a flow meter. Next best: read the maker’s pump curve at your actual head height, picking the closest pipework setup. Worst: use the maximum flow without allowing for lift.
What wavemakers are for
Flow inside the display should keep detritus moving towards your filtration, refresh the water around your rockwork, and avoid one harsh jet. Start gently, aim across or along the rocks rather than straight at corals, and only use pulse or random modes if they actually improve flow in your layout.
A simple worksheet
| What you need | Example | Why it matters |
|---|---|---|
| Net water volume | 210L | Use the real water in your system, not the advertised size. |
| Return flow at the outlet | 1,050L/h | Use the flow left after lift and plumbing losses. |
| Return turnover | 5×/hour | Flow ÷ volume. |
| Display circulation | Depends on your layout | A wavemaker’s rating isn’t a guarantee of useful flow. |
THE SHORT ANSWER
Work out return flow and display flow separately
Turnover is flow divided by water volume, but one turnover figure can hide two very different jobs. Your return pump moves water through the sump and back, pushing against head pressure. Your wavemakers move water around the rocks and livestock inside the display. Adding their headline litres per hour together doesn’t accurately describe either.
Use your real display volume for planning circulation, and your whole system’s water for the bigger picture. Treat turnover ranges based on livestock as a starting point, then fine-tune by watching the sand, how corals respond, surface movement and where detritus collects. What you want is flow that reaches everywhere, not the biggest multiple.
STEP BY STEP
How to do it
- 01
Use the right water volume
Say whether you’re using the tank’s label size, the real display volume or the whole system. Rock and water levels can change the figure a lot.
- 02
Work out your real return flow
Use the pump curve after allowing for lift and plumbing losses. A rating measured with no lift overstates what reaches the outlet.
- 03
Work out display circulation
Use the setting your wavemakers actually run at, not necessarily their maximum. Keep this figure separate from the return.
- 04
Check it in the tank
Watch the surface, where debris goes, the sand and your livestock. If you see a strong jet in one spot and a dead zone elsewhere, move the pumps before turning them up.
YOUR SITUATION
What to do in your situation
Moderate, broad movement reaches most areas
Start at the lower end and tune for gentle variety.
Different animals need different flow, and rocks create sheltered spots
Create areas of high, medium and lower flow rather than one average.
Colonies and branches block circulation
Use several adjustable pumps, and expect to move them as corals grow.
COMMON MISTAKES
Mistakes to avoid
Adding up every pump’s rating
Return pump and wavemaker figures describe different conditions and can’t simply be added together.
Using the label litres
Using the tank’s advertised size makes turnover look lower than it really is.
Fixing dead spots with more power
Turning pumps up can make a harsh jet worse while the spot behind the rock stays still.
WORKED EXAMPLE
A 150-litre display with two adjustable pumps
After rock and sand, the display holds about 125 litres of water. Two wavemakers are each rated at 4,000 l/h but run at about half power, giving a rough circulation figure of 4,000 l/h, or 32 times the display volume. That doesn’t mean every part of the tank gets 32 equal turnovers.
If detritus still gathers behind the main rock island while sand shifts at the front, more power isn’t automatically the answer. Re-angle one pump, try an alternating pattern or move a pump so water reaches behind the rocks, then look again.
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
What turnover does a reef tank need?
Livestock-based ranges are only a starting point. Your tank’s shape, rockwork, pump pattern and coral placement decide what flow is useful.
Does return pump flow count?
It adds some movement, but work it out and size it for the sump, rather than relying on it to move water around the display.
Can a reef tank have too much flow?
Yes. Too much speed in one spot can blow sand around, damage coral tissue or stop animals feeding, even if the average looks fine.