Reef equipment sizing reference tool
Protein Skimmer Sizing Calculator & Fit Checker
Compare the exact manufacturer's heavy, medium or light load rating with the volume basis the maker actually uses, then check mount type, water depth, footprint and cup-removal clearance.
This calculator uses no universal protein-skimmer oversizing multiplier. Its operating water estimate is a planning aid and does not become a manufacturer capacity rating. When a maker says a rating is excluding sump, choose the display-only basis.
Manufacturer-tier and installation fit checker
Check skimmer capacity evidence and physical fit
Enter your aquarium, the exact manufacturer's published load tiers, and the installation dimensions you can verify. The calculator keeps the water-volume estimate separate from the manufacturer rating basis.
Enter the manufacturer's exact rating and any dimensions you can verify, then select Check skimmer fit.
Why this calculator does not multiply tank volume by one generic factor
Current protein-skimmer manufacturers do not use one standardized capacity method. CoralVue publishes separate heavy, medium and light bioload tiers for products such as Reef Octopus and IceCap. TUNZE uses load-dependent seawater ranges. Red Sea can publish SPS, mixed-reef and fish-only system categories. A single cross-brand multiplier would erase those distinctions.
The calculator therefore asks you to enter the exact current manufacturer values. If the manufacturer rating basis is unclear, select manufacturer-defined. The result will refuse to invent an automatic capacity verdict.
Operating water estimate and manufacturer capacity are different numbers
Display volume plus working sump volume can be useful for planning the system's approximate water inventory. Rock, sand, equipment and air gap reduce actual operating water, so the calculator lets you enter a transparent displacement percentage.
That estimate is not automatically the number a skimmer manufacturer used. If the documentation says the rating is excluding sump, the calculator compares the display volume with the selected manufacturer tier instead. If a maker explicitly rates total system water, choose that basis. If the basis is not documented, use manufacturer-defined and verify manually.
If you need a more detailed aquarium-volume estimate first, use the aquarium volume calculator.
Manufacturer examples: preserve the label, do not normalize it away
The current IceCap K1-100 documentation publishes 40 gal heavy / 60 gal medium / 80 gal light, excluding sump. Reef Octopus Classic 100 HOB publishes 55 gal heavy / 75 gal medium / 105 gal light, also excluding sump. TUNZE 9004 DC instead publishes a 15 to 79 USgal range depending on tank load.
Those examples show why “rated for X gallons” is incomplete unless the load category and rating basis stay attached. Use the protein skimmer guide for a broader comparison of current skimmer architectures and documented rating schemes.
Capacity fit cannot rescue a bad installation
An in-sump skimmer is not a fit when the system has no sump. A footprint match can still fail if plumbing or the pump cannot be removed for cleaning. A skimmer that physically fits under a cabinet can still be impossible to service if there is no room to lift the collection cup.
The physical checks on this page are intentionally separate: mount architecture, operating water depth, rectangular footprint and vertical service clearance. A pass on one does not cancel a failure on another. After fit is confirmed, use the protein skimmer setup and tuning guide to establish the manufacturer water depth, break-in and baseline foam level. If a correctly sized installation later becomes unstable, use the protein skimmer troubleshooting guide to separate water-depth, venturi, additive, tuning and pump causes.
How the same manufacturer evidence changes at 40 and 75 gallons
The 40-gallon breeder protein skimmer guide shows a case where the IceCap K1-100 lands exactly on a documented 40-gallon heavy-bioload boundary, while the Reef Octopus Classic 100 HOB keeps a 40-gallon display below its 55-gallon heavy tier.
The 75-gallon protein skimmer guide shows why the same HOB model becomes only a medium-bioload match at 75 gallons, while a larger in-sump model provides documented heavy-load headroom. The tank size did not change the manufacturer's table; it changed where the aquarium sits inside that table.
CO2 scrubbers add an air-path constraint after sizing
A skimmer that fits the tank and sump can behave differently when its venturi air path is routed through absorbent media, long tubing or recirculation plumbing. Added restriction and moisture can alter air draw and foam behavior.
Use the reef CO2 scrubber guide to diagnose indoor-CO2 contribution first and to check airflow, condensation and recirculation boundaries. Do not treat a scrubber as part of the capacity rating itself.
Methodology and sources
PetGearReport has not physically tested your skimmer, measured your installed water volume or validated the fit of your cabinet. This tool performs transparent arithmetic and fit checks from the values you enter. The exact manufacturer's current documentation remains authoritative for model capacity, mounting and operating depth.
- CoralVue — IceCap K1-100 — 40/60/80 gallon heavy/medium/light tiers, excluding sump, footprint and operating-depth documentation.
- CoralVue — Reef Octopus Classic 100 HOB — 55/75/105 gallon heavy/medium/light tiers, excluding sump, and HOB installation context.
- TUNZE — Comline DOC Skimmer 9004 DC — load-dependent 15–79 USgal guidance and in-tank/rear-chamber installation data.