Protein Skimmer Size Calculator
Size a protein skimmer for your saltwater aquarium from tank volume, bioload, and feeding level.
Returns recommended skimmer rating in gallons per hour.
A protein skimmer (also called a foam fractionator) removes dissolved organic compounds from saltwater aquariums before they break down into ammonia and nitrate. It works by injecting fine air bubbles into a column of water — organic molecules stick to the bubbles and are carried up into a collection cup as dark, smelly “skimmate.”
Skimmer Sizing Rule of Thumb
Rated skimmer capacity = Actual tank volume × Bioload multiplier
| Bioload Level | Multiplier | Typical Setup |
|---|---|---|
| Light | 1.0× | Soft corals, few fish, light feeding |
| Moderate | 1.5× | Mixed reef, moderate fish stocking |
| Heavy | 2.0–2.5× | SPS dominant, heavy fish load, heavy feeding |
| Fish-only aggressive | 2.5–3.0× | Large predator fish, messy eaters |
For a 100-gallon mixed reef with moderate bioload: choose a skimmer rated for at least 150 gallons.
Air Draw Rate
The skimmer’s air draw (measured in liters per hour, L/hr) is a better performance indicator than the manufacturer’s gallon rating:
| Tank Volume | Recommended Air Draw |
|---|---|
| 20–50 gallons | 200–400 L/hr |
| 50–100 gallons | 400–800 L/hr |
| 100–200 gallons | 800–1500 L/hr |
| 200–400 gallons | 1500–3000 L/hr |
| 400+ gallons | 3000+ L/hr |
Skimmer Body Diameter
Wider skimmer bodies provide longer contact time between bubbles and water. A general guideline:
Body diameter (inches) ≈ 1.5 + (Tank gallons / 100) × 2
For a 150-gallon tank: 1.5 + (150/100) × 2 = 4.5" body — a 4"–5" body skimmer is appropriate.
Worked Example — 120-Gallon SPS Reef Tank
Bioload: Heavy (SPS corals, 15 fish, heavy feeding). Multiplier: 2.0×. Target skimmer rating: 120 × 2.0 = 240 gallons. Recommended air draw: ~1200 L/hr. Body diameter: 1.5 + (120/100) × 2 = 3.9" — look for 4"+ body.
Choose a skimmer rated for 200–300 gallons with a needle-wheel or mesh-wheel pump drawing 1000–1500 L/hr of air. Place in a sump with a stable water level (skimmers are sensitive to water level fluctuations).
Break-In Period
New skimmers take 1–3 weeks to “break in.” During this time the acrylic walls are smooth and bubbles slide off instead of forming a stable foam column. The skimmer may overflow or produce wet, clear skimmate initially. This is normal. Do not adjust the settings repeatedly — let it stabilize.
Skimmer Types
| Type | Pros | Cons |
|---|---|---|
| Hang-on-back (HOB) | Easy install, no sump needed | Limited to small tanks (<75 gal) |
| In-sump | Best performance, hidden | Requires sump |
| Internal | No sump needed, decent output | Takes up tank space |
| External | Highest air draw, large systems | Complex plumbing |
Maintenance
Clean the collection cup and neck weekly. A dirty neck reduces foam production. Deep-clean the pump impeller and air silencer monthly. Replace the silencer every 6–12 months.
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This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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