Six things to check
Estimate the actual water once rock, sand, equipment and the sump’s running level are accounted for, not the tank’s advertised litres.
Use the maker’s heavy-load or SPS rating where there is one. The number and size of your fish and how much you feed matter more than coral labels.
Decide whether it’s your main way of removing waste, backing up a refugium, or mainly there for oxygen and a safety margin.
Measure the exact chamber, including baffles, pipework and where cables come out. Leave room for your fingers and for taking it apart.
Check the sump depth it needs. A stand can raise a skimmer, but it can’t give you more cabinet height.
Add the height you need to lift and tilt the cup out under cabinet braces or shelves.
A simple rule for reading ratings
If the maker publishes ratings for different loads, pick the one that matches how you’ll stock and feed, then choose a model where your net water volume sits comfortably inside that rating, not right at its most optimistic upper limit. If there’s only one maximum figure, use it to compare models, not as a guarantee.
Example: a tank sold as 300 litres
Once you allow for rock, sand and equipment, the real water may be nearer 240–260 litres. A moderately stocked mixed reef doesn’t automatically need a “600-litre” skimmer. Start with the maker’s load assumptions, then check the footprint, water depth, air draw and room to maintain it. An adjustable skimmer that fits properly can be more useful than a bigger AC model squeezed in at the wrong depth.
Measure these before you shop
| Measurement | Note down | Why it matters |
|---|---|---|
| Chamber length × width | In millimetres | Has to fit the pump and outlet direction, not just the body’s diameter. |
| Water depth | Normal, and the maximum during a power cut | Decides where the skimmer sits and the risk of overflowing. |
| Floor to the nearest obstruction | In millimetres | Decides whether you can lift the cup out. |
| Type of control | AC, DC or app | Changes how much you can adjust it, whether you need a controller, and the cabling. |
Don’t compare on air draw alone
Litres of air per hour can help compare models within the same range, but the body design, pump tuning, water flow and stability matter too. We list air draw where the maker publishes it; we don’t turn it into a made-up performance score.
THE SHORT ANSWER
A skimmer has to fit your tank’s load and your sump
Start with your real net water volume, then adjust for feeding and stocking, rather than buying against a maker’s biggest capacity claim. Makers use different assumptions, so ratings are useful for comparing within one brand but poor as a universal measure. Air intake, pump power and neck size add context, but none of them predicts performance on its own.
Physical fit matters just as much. Measure the sump footprint, the water depth it needs, its total height, room to lift out the cup, cabinet braces and access to the pump. A skimmer that’s perfect on paper but can’t be removed, cleaned or kept at a steady water depth is the wrong size.
STEP BY STEP
How to do it
- 01
Work out your net water volume
Add the display and the sump’s running water, then take off a sensible estimate for rock, sand and air gaps. Note how uncertain the figure is rather than pretending it’s exact.
- 02
Be honest about your load
Use your fish’s adult size, how you feed and your nutrient trend. Allow extra margin for heavy feeding, but don’t automatically double every rating.
- 03
Measure the space you have
Check the footprint in each allowed orientation, the recommended water depth, overall height and the space needed to lift out the cup. Include pipework and cabinet doors.
- 04
Check servicing and control
Make sure impellers, pumps and seals are available, how it’s adjusted, the drain options, and whether advertised features need a separate controller.
YOUR SITUATION
What to do in your situation
Light feeding and a modest nutrient trend
Choose close to a cautious rating rather than massively oversizing.
Frequent feeding and a growing waste load
Allow a bigger capacity margin, while making sure it can still run steadily.
Footprint or cup clearance is what limits you
Pick a compact model designed for your measured depth, not a bigger one forced into the space.
COMMON MISTAKES
Mistakes to avoid
Trusting a single tank rating
A capacity range may mix light and heavy loads, and brands don’t define those terms the same way.
Forgetting about steady water depth
Evaporation shifts how the skimmer runs unless the chamber is baffled or you have an auto top-off.
Ignoring how the cup comes out
Overall height isn’t enough; the cup usually needs extra room to lift or slide out for cleaning.
WORKED EXAMPLE
Sizing for a 200-litre sump system
The display is sold as 170 litres and the sump holds 50 litres when running, but rock, sand and the sump’s air gap bring the real water down to about 175 litres. The tank will have several well-fed fish, so a model whose cautious heavy-load rating covers roughly that volume is a better start than one relying on its maximum light-load claim.
The cabinet then rules out two options: one needs deeper water than the chamber has, and the other leaves just 5mm above the cup. The remaining model has a workable stand height, room for servicing and UK pump parts. Fitting physically is what makes it the right size.
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
Should I buy a skimmer rated for twice my tank?
Not as a hard rule. Use the brand’s load assumptions, your net volume and your feeding, and avoid one so big it can’t keep a steady foam head.
Does more air intake mean better skimming?
It’s useful context, but not a measure of quality on its own. The body design, neck, water flow and tuning all matter.
Can I put a skimmer on a stand?
Often, yes, if the maker allows it, as long as the stand is stable and you still have room to lift the cup out.