Why three baffles instead of two?

Two baffles work for skimmer-only sumps but leak bubbles into a refugium or return chamber. The third baffle catches anything that escapes the first two. The 1″ gaps are critical — narrower traps bubbles, wider lets them through.

Does the bubble trap orientation matter?

Yes. Down-up-down or up-down-up both work; the point is alternating flow direction. Most builds put the first baffle floor-mounted (water flows over), the middle baffle floating (water flows under), and the last baffle floor-mounted again. Same effect either order.

Can I use acrylic instead of glass for baffles?

Yes, but silicone doesn't bond to acrylic the way it bonds to glass. Use Weld-On 4 for acrylic-to-acrylic, or accept that glass baffles in a glass sump is the safer build.

How much rundown volume do I need in the return chamber?

When the power kicks off, water in the return plumbing drains back into the sump (drain plumbing keeps draining too, until pumps stop siphoning). Most builds see 1–3 gal of rundown. Make sure the return chamber's normal water level + rundown stays below the top of the sump. Test it.

Calculator

DIY Reef Sump Baffle Calculator

Name: DIY Reef Sump Baffle Calculator
Author: reefcalcs

A good reef sump turns mess into clean water. A bad one makes microbubbles, runs the return dry, and floods the skimmer chamber every time a powerhead burps. The geometry that separates the two is mostly baffle placement: chamber widths sized for residence time, bubble traps tight enough to actually trap bubbles, and a return chamber large enough to hold the rundown volume when the power kicks off.

JavaScript is disabled in your browser, so the interactive calculator below won't run. The math, formula, and worked example are all written out further down this page under “How this is calculated.” Enable JS (or open this page in another browser) to use the live calculator.

Whether you compute by hand or in the calculator, the same caveat applies: verify against your test kit and the linked primary source before dosing. reefcalcs is reference information, not veterinary or engineering advice.

Sump baffle layout

Sump length, inside (in)

Sump width, inside (in)

Sump height, inside (in)

Running water height (in)

Water depth with pumps on.

Display tank (US gal)

Layout

Skimmer section width

15.6in

Refugium section width

4.5in

Return section width

10.4in

Bubble-trap span

2.75in

3 baffles, 1″ gaps, ¼″ glass.

Baffle gap

1.00in

Last baffle height

7.0in

3″ below running surface.

Skimmer chamber volume

10.8gal

Refugium volume

3.1gal

Return chamber volume

7.2gal

Section sizing targets: skimmer = ~12 % of display volume (for residence time), return = ~8 % of display (for ATO dwell). Bubble trap = 3 baffles with 1″ gaps. Last baffle sits 3″ below running surface. Source: Melev's Reef sump-design guide.

How this is calculated

Section widths from target volumes:
  gal_per_inch_along_length = (width_in × running_height_in) ÷ 231

  target_skimmer_gal  = display_gal × 0.12
  target_return_gal   = display_gal × 0.08
  refugium_in         = sump_length − skimmer − return − 2 × bubble_trap

Bubble trap (3 baffles, 1″ gaps, ¼″ glass):
  span = 3 × 0.25 + 2 × 1.0 = 2.75 in

Baffle heights:
  first baffle:  full height (sits on floor, water flows over)
  middle:        starts ~2 in above floor (water flows under)
  last baffle:   sits 3 in below running water surface

The 12 %/8 % split is the Melev's Reef formula and it works for most builds. For tanks with lots of evaporation (open-top, basement, no glass top) bump return-chamber size to 10 % to absorb a longer ATO dwell. For SPS-dominant builds with oversized skimmers, you can drop skimmer-chamber to 10 % and let the skimmer cup sit lower.

Silicone choice matters. Use GE Silicone I (clear) or Momentive RTV108 — both are 100 % silicone, no anti-mildew additives, no acetic acid that fish hate during curing. Mask, run a bead, tool with a finger, peel mask immediately. Cure 7 days minimum before filling. Aquarium-rated silicone holds 50-gal vertical baffles indefinitely; cheap caulk fails in months.

FAQ

Why three baffles instead of two?: Two baffles work for skimmer-only sumps but leak bubbles into a refugium or return chamber. The third baffle catches anything that escapes the first two. The 1″ gaps are critical — narrower traps bubbles, wider lets them through.
Does the bubble trap orientation matter?: Yes. Down-up-down or up-down-up both work; the point is alternating flow direction. Most builds put the first baffle floor-mounted (water flows over), the middle baffle floating (water flows under), and the last baffle floor-mounted again. Same effect either order.
Can I use acrylic instead of glass for baffles?: Yes, but silicone doesn't bond to acrylic the way it bonds to glass. Use Weld-On 4 for acrylic-to-acrylic, or accept that glass baffles in a glass sump is the safer build.
How much rundown volume do I need in the return chamber?: When the power kicks off, water in the return plumbing drains back into the sump (drain plumbing keeps draining too, until pumps stop siphoning). Most builds see 1–3 gal of rundown. Make sure the return chamber's normal water level + rundown stays below the top of the sump. Test it.

Where to buy

Pick a skimmer rated for your display + sump volume; the skimmer-chamber sizing in this calculator assumes the skimmer cup sits inside, not external.

Bubble Magus Curve 5 skimmer

Best budget skimmer for tanks up to 80 gal.

View on Amazon →

Reef Octopus Classic 110-INT skimmer

Mid-range workhorse. In-sump, 80–150 gal.

View on Amazon →

Affiliate links. As an Amazon Associate, reefcalcs earns from qualifying purchases.

Share / embed this calculator

Paste this snippet into any blog or forum post that allows raw HTML to embed the calculator.

<iframe
  src="https://reefcalcs.com/embed/sump-baffles/"
  width="100%"
  height="720"
  style="border: 1px solid #1C2D34; border-radius: 8px; max-width: 720px;"
  loading="lazy"
  title="ReefCalcs DIY Reef Sump Baffle Calculator"
></iframe>

Sources & references

01
Melev's Reef — Sump Design Guide (chamber-sizing rules)
https://www.melevsreef.com/sump-design
02
Bulk Reef Supply — 5 Minute Saltwater Aquarium Guide Ep 27: Sumps
https://www.bulkreefsupply.com/content/post/5-minute-saltwater-aquarium-guide-ep27-sumps
03
NIST Handbook 44 App. C — 231 in³ per US gallon (volume conversion basis)
https://www.nist.gov/pml/owm/publications/handbook-44

Last reviewed 2026-05-25