Aquarium · Reef equipment
Protein skimmer sizing calculator
A protein skimmer pulls dissolved organic waste out of saltwater before it breaks down into ammonia and nitrate, and undersizing one is one of the more common reef tank mistakes. The tricky part is that manufacturer gallon ratings assume a lightly stocked tank, which is rarely what you actually have. This calculator scales the rated capacity you need to your real bioload, not just your tank size.
Minimum rated capacity
94–113 gal
Based on display volume, not total system volume including sump.
Target air draw
75–150 L/hr
A skimmer's real air draw matters more than its gallon rating alone, since manufacturer ratings are often optimistic.
What to actually buy
Avast Marine Works Swabbie Protein Skimmer Neck Cleaner
A correctly sized skimmer still underperforms once gunk builds up inside the neck, which happens fast and often gets ignored. This motorized cleaner wipes the neck automatically a few times a day on a timer, keeping actual performance closer to the rated capacity this calculator points you toward.
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How this is calculated
Manufacturer skimmer ratings are typically based on an ideal, lightly stocked tank, so buying a skimmer rated for exactly your tank's gallon count usually undersizes it in practice. For a light bioload (fish-only with live rock), a skimmer rated for your actual display volume is reasonable. For a mixed reef, Bubble-Magus and SaltTankReport's sizing guides both recommend a skimmer rated for 1.25 to 1.5 times your display volume. For a heavy bioload, SPS-dominant tank with a lot of fish or feeding, size up to 1.5 to 2 times your display volume.
Sizing is based on display tank volume specifically, not total system volume including the sump, since that's where the real bioload concentrates. A skimmer sized off your total system volume including an oversized sump will come out undersized relative to what's actually producing waste.
Air draw, how many liters of air the skimmer actually pulls through per hour, is a better real-world performance signal than the gallon rating alone, since gallon ratings vary in how honestly different manufacturers set them. This calculator gives a target air draw of 1 to 2 liters of air per gallon of display volume per hour, so you can sanity-check a specific skimmer's spec sheet against your tank rather than relying on its marketed gallon rating by itself.
Sources
Frequently asked questions
Should I size up or down within the recommended range?
Size toward the high end if you feed heavily, have a lot of fish, or run a mixed reef leaning toward SPS. Size toward the low end for a lightly stocked tank with modest feeding. For most mixed reef tanks, the middle of the range is a reasonable default.
My skimmer is rated correctly but isn't producing much skimmate, what's wrong?
A dirty skimmer neck is the most common cause, buildup inside the neck restricts the foam column and tanks performance even on a correctly sized unit. Clean the neck regularly, and check that your water level and air intake are set per the manufacturer's instructions, both affect performance as much as sizing does.
Do I need a bigger skimmer if I add more fish later?
Possibly, since bioload is the input that actually drives the sizing math here, not just tank volume. If your stocking level moves from light to medium or medium to heavy, recheck this calculator with the new bioload setting rather than assuming the original sizing still holds.
Is a bigger skimmer always better?
Not necessarily. An oversized skimmer for a lightly stocked tank can strip out too many trace elements and nutrients corals actually use, sometimes called "over-skimming." Sizing to your real bioload, not just buying the largest skimmer that fits your sump, gets better results than going as big as possible.