The six checks
Estimate actual water after rock, sand, equipment displacement and the sump operating level—not the display’s advertised litres.
Use the maker’s heavy-load or SPS band where published. Fish number, body mass and feeding matter more than coral labels alone.
Decide whether the skimmer is primary export, supporting a refugium, or mainly providing aeration and resilience.
Measure the exact skimmer chamber, including baffles, pipework and cable exits. Allow room for fingers and disassembly.
Compare the required sump depth. A stand can raise a skimmer; it cannot create cabinet height.
Add the height needed to lift and tilt the collection cup beneath cabinet braces or shelves.
A practical rating rule
If a manufacturer publishes load bands, place the system in the band that matches its intended stocking and feeding, then choose a model where your net volume sits comfortably inside that band—not exactly at the most optimistic upper limit. If only one maximum figure is published, treat it as a comparison clue rather than a guarantee.
Example: a nominal 300-litre system
After displacement, the operating water may be closer to 240–260 litres. A moderately stocked mixed reef does not automatically need a “600-litre” skimmer. First compare the manufacturer’s load assumption, then verify footprint, water depth, air draw and maintenance access. A correctly fitted controllable model may be more useful than a larger AC unit squeezed into the wrong depth.
Measurements to save before shopping
| Measurement | Record | Why it matters |
|---|---|---|
| Chamber length × width | Millimetres | Must include pump and outlet orientation, not body diameter alone. |
| Water depth | Normal and power-off maximum | Controls operating position and overflow risk. |
| Floor to obstruction | Millimetres | Determines whether the cup can be removed. |
| Desired control | AC, DC or app | Changes adjustment range, controller cost and cable planning. |
Do not compare air draw alone
Litres of air per hour can help compare models within a range, but reaction-body design, pump tuning, water throughput and stability also matter. We list air draw when a manufacturer publishes it; we do not convert it into an invented performance score.