Protein Skimmer Size Calculator

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

Published ranges are broader than that (heavy is usually quoted as 2.0 to 2.5, fish-only as 2.5 to 3.0). Single figures are used here so the arithmetic is reproducible, and they sit at the conservative end of each band.

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, because the gallon rating is whatever the marketing department decided:

Tank volume picks the band. Bioload decides where inside it you land, which is why a lightly stocked 120-gallon softie tank wants the 800 end and a heavily fed SPS system of the same size wants 1,200.

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", so a 4" to 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. Air draw: the 100 to 200 gallon band is 800 to 1500 L/hr, and a heavy bioload sits about 57% of the way up it, so ~1,200 L/hr. Body diameter: 1.5 + (120/100) × 2 = 3.9", so look for a 4"+ body.

Choose a skimmer rated for roughly 215 to 310 gallons with a needle-wheel or mesh-wheel pump drawing about 1,200 L/hr of air. Place it in a sump with a stable water level, because skimmers are sensitive to level fluctuations and an inch of drift turns a good foam column into an overflowing cup.

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. Resist the urge to keep adjusting it, because every change restarts the clock. Set it once and leave it alone for a fortnight.

Skimmer Types

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.