How do I know my pH problem is from indoor CO₂?

Stand near an open window for an hour with the skimmer intake hose, then check pH. If it climbs 0.2–0.3 within a few hours, indoor CO₂ is the culprit. A cheap NDIR CO₂ meter (Aranet4, CO2Click) confirms it directly.

Is Sofnolime safe around fish?

The pellets themselves are caustic if they touch water — keep the scrubber column dry. The air leaving the scrubber is just CO₂-depleted ambient air; nothing harmful goes into the tank.

Why does my media die so fast?

Either your skimmer pulls more air than you think (check spec sheet) or your room CO₂ is high all the time (cooking, gas range, occupants in a sealed home). Track both. The single biggest fix is opening a window during cooking and at night.

How does this differ from a refugium for pH?

A refugium on reverse photoperiod consumes CO₂ via macroalgae photosynthesis — same effect, no consumable, but slower. Both work. Stack them if you can.

Calculator

CO₂ Scrubber Sizing Calculator

Name: CO₂ Scrubber Sizing Calculator
Author: reefcalcs

Reef pH is set by the CO₂ concentration of the air your skimmer pulls. In a sealed modern home with occupants, indoor CO₂ runs 800–2000 ppm — vs 420 ppm outside — and that 2-to-5× difference is exactly enough to drop tank pH from 8.2 to 7.7. A CO₂ scrubber (Sofnolime / Sodasorb soda lime) on the skimmer intake removes the excess. This calculator predicts how long a given media volume lasts.

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.

CO₂ scrubber sizing

Skimmer air draw (L/min)

Most reef skimmers pull 200–800 L/min.

Indoor CO₂ (ppm)

Use a CO₂ meter; sealed homes range 800–2000 ppm.

Scrubber media (litres of Sofnolime/Sodasorb)

Run hours per day

CO₂ absorbed per day

561.6L

Media expected to last

—days

Watch the colour indicator; replace before it fully shifts.

Grams CO₂ removed per day

1110g

⚠ Media expected to exhaust in <2 weeks. Either source more media, lower skimmer air, or fix the room-air problem at its source (ventilation).

Capacity assumption: 120 L CO₂ per L of Sofnolime 797 / generic soda lime (Molecular Products datasheet). Real-world life is longer because room CO₂ drops at night and during ventilation.

How this is calculated

Air drawn per day:
  air_L/day = skimmer_L/min × run_min/day

CO₂ above outdoor baseline (420 ppm):
  delta_ppm = max(0, indoor_ppm − 420)

CO₂ absorbed per day:
  CO₂_L/day = air_L/day × delta_ppm / 1,000,000

Media life:
  days = (media_L × 120 L CO₂/L media) ÷ CO₂_L/day

Capacity: 120 L CO₂ per L of Sofnolime / Sodasorb (Molecular Products).

The colour indicator on Sofnolime / Sodasorb shifts from white to purple as the alkali exhausts. Pull and replace before the entire column turns — partial-exhaustion media still has capacity but breakthroughs intermittently. Don't try to regenerate it; just buy fresh.

The cheaper fix is ventilation. A 4″ duct from outside to the skimmer cabinet (with a HEPA pre-filter) gives unlimited fresh-air supply and skips the consumable entirely. Scrubbers make sense when an outside duct isn't possible — apartments, basement fishrooms with bad outside air, or sealed equipment closets.

FAQ

How do I know my pH problem is from indoor CO₂?: Stand near an open window for an hour with the skimmer intake hose, then check pH. If it climbs 0.2–0.3 within a few hours, indoor CO₂ is the culprit. A cheap NDIR CO₂ meter (Aranet4, CO2Click) confirms it directly.
Is Sofnolime safe around fish?: The pellets themselves are caustic if they touch water — keep the scrubber column dry. The air leaving the scrubber is just CO₂-depleted ambient air; nothing harmful goes into the tank.
Why does my media die so fast?: Either your skimmer pulls more air than you think (check spec sheet) or your room CO₂ is high all the time (cooking, gas range, occupants in a sealed home). Track both. The single biggest fix is opening a window during cooking and at night.
How does this differ from a refugium for pH?: A refugium on reverse photoperiod consumes CO₂ via macroalgae photosynthesis — same effect, no consumable, but slower. Both work. Stack them if you can.

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/co2-scrubber/"
  width="100%"
  height="720"
  style="border: 1px solid #1C2D34; border-radius: 8px; max-width: 720px;"
  loading="lazy"
  title="ReefCalcs CO₂ Scrubber Sizing Calculator"
></iframe>

Sources & references

01
Molecular Products — Sofnolime 797 datasheet (capacity 120 L CO₂ per L media)
https://www.molecularproducts.com/products/sofnolime-797-medical/
02
Randy Holmes-Farley — CO₂, pH, and the marine aquarium
http://reefkeeping.com/issues/2002-04/rhf/feature/index.php
03
Bulk Reef Supply — Why Does My pH Drop in Winter?
https://www.bulkreefsupply.com/content/post/why-does-my-ph-drop-in-the-winter

Last reviewed 2026-05-25