A protein skimmer strips dissolved organic waste out of the water before it breaks down into nitrate and phosphate. Soft-coral and fish-only tanks can skip it; LPS and SPS tanks need one. The hard part isn't picking a brand — it's picking one that is rated correctly and physically fits your sump, which is where most first skimmers go wrong.
The ReefRig Skimmer Method
Three checks, in order:
- Rating: the skimmer's rated capacity should be at least
system volume × bioload × 1.5, up to about 3× (oversized skimmers won't produce stable foam on a light bioload). Bioload is roughly 1.0 for light stocking, 1.3 for medium, 1.6 for heavy. - Footprint: the pump body's length × width must fit inside your sump's skimmer chamber, with room to lift the cup off.
- Water depth: every in-sump skimmer is tuned for a rated water level. Run it too deep or too shallow and it skims wet or dry. Match the chamber's water depth to the skimmer's rated operating range — or raise the skimmer on a riser.
RULE OF THUMB
A 90-gallon mixed reef at medium bioload (90 × 1.3 × 1.5 ≈ 175) needs a skimmer rated for roughly 175 gallons — but a correctly-rated skimmer still under-performs if its body won't fit your sump chamber or sits at the wrong water depth. Size all three before you buy.
Compare: in-sump and HOB skimmers
Ratings below are for heavy bioload — the conservative reef number. Each skimmer is rated far higher for lightly-stocked or fish-only tanks. All prices verified July 2026.
| Skimmer | Rated (heavy) | Footprint L×W | Rated depth | Price |
|---|---|---|---|---|
| IceCap K1-100 | 40 gal | 4.5×7.5 in | 6–8 in | $155.91 |
| AquaMaxx HOB-1.5 (hang-on) | 40 gal | 7.5×3.5 in | hangs on rim | $219.99 |
| Bubble Magus Curve 5 | 55 gal | 7.28×7.08 in | 9.5–11 in | $151.99 |
| Reef Octopus Classic 110INT | 60 gal | 10.2×6.9 in | 7–8 in | $326.85 |
| Nyos Quantum 160 | 75 gal | 7.1×9.8 in | 8.6 in | $449.99 |
| Reef Octopus Classic 150INT | 110 gal | 12.4×8.7 in | 6–8 in | $381.14 |
| Nyos Quantum 220 | 125 gal | 9.3×12.2 in | 8 in | $639.99 |
Nyos publishes a single recommended water level, not a range. The AquaMaxx HOB hangs on a tank or AIO rear-chamber rim, so it has no sump-depth spec — good for AIO tanks with no sump.
Picks by tank size
- Nano / AIO (up to ~40 gal): most run skimmerless on softies. If you want one, the AquaMaxx HOB-1.5 ($219.99) hangs on an AIO rear chamber, or the IceCap K1-100 ($155.91) fits a small sump.
- 40–75 gal: Bubble Magus Curve 5 ($151.99) is the value pick; step up to the Reef Octopus Classic 110INT ($326.85) for a sturdier pump.
- 75–120 gal mixed / LPS: Reef Octopus Classic 150INT ($381.14) or Nyos Quantum 160 ($449.99).
- 120 gal-plus SPS: Nyos Quantum 220 ($639.99), rated to 125 gallons at heavy bioload and far more when lightly stocked.
The Reefer G3 depth trap (check this before you buy)
Here's a fit problem that catches people upgrading to a modern sump system. Red Sea's current Reefer G3 tanks have deep skimmer chambers: the Reefer 350 G3's chamber holds about 9.2 inches of water and the 625 G3's holds about 11 inches. But most popular in-sump skimmers are tuned for a shallower level — the Reef Octopus Classic runs at 6–8 inches, the Nyos Quantum 220 at 8 inches.
Drop one of those into a Reefer G3 chamber and it sits too deep, skimming wet and overflowing the cup. The fix is a riser (a stand under the skimmer) to bring the pump up to its rated water line — or picking a skimmer rated for a deeper level, like the Bubble Magus Curve, which is tuned for 9.5–11 inches. It's a five-minute check that saves a week of frustration, and it's exactly the kind of thing the depth column above exists for.
ORIGINAL FINDING
Reefer G3 skimmer chambers run 9.2–11 inches deep, while most mainstream in-sump skimmers are rated for ≤8–9 inches. In a deep chamber, put the skimmer on a riser — or choose one rated for the deeper level — or it skims too wet.
Bottom line
Rate the skimmer with the ReefRig Skimmer Method, then measure your sump chamber before you check out. A perfectly-rated skimmer that doesn't fit is a return shipment. To have your whole build sized — skimmer included — against your exact tank and coral type, use the System Builder.
Ratings, footprints, water-depth ranges and prices verified July 2026 against manufacturer and distributor spec tables (Bulk Reef Supply, CoralVue, SaltwaterAquarium.com, Premium Aquatics). Heavy-bioload ratings shown; lightly-stocked ratings are higher. Re-check pricing before purchase.