Protein Skimmer Size Calculator
Size a protein skimmer for your saltwater aquarium.
Match skimmer capacity to tank volume, bioload, and feeding level.
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.