Vinegar dosing in a reef tank, sourced

Why carbon dosing

Reef tanks accumulate nitrate (NO₃) and phosphate (PO₄) from fish food, fish waste, and the bottom of the food chain. Skimming exports some of it. Macroalgae (chaeto in a refugium) exports more. But in heavily-fed tanks — especially SPS systems with fat-loaded coral foods — the nutrients still climb. Carbon dosing is the export multiplier.

The mechanism: heterotrophic bacteria — the ones that live on dissolved organic carbon, not on photosynthesis — are normally carbon-limited in seawater. Add a carbon source (acetate from vinegar, ethanol from vodka, sucrose from sugar) and the bacteria multiply. To build their biomass they need nitrogen and phosphorus in a roughly Redfield ratio (106 C : 16 N : 1 P). They pull NO₃ and PO₄ from the water to do it. Then the skimmer or filter sock removes the bacterial floc. Net nutrient export.

Without skimming or mechanical filtration, carbon dosing doesn't work — the nutrients just cycle inside the bacteria. You need both the carbon supply and the export step.

The starting dose

Randy Holmes-Farley's original methodology article gives the starting dose: 0.1 mL of distilled white vinegar (5% acetic acid) per US gallon of system water per day. So a 100-gallon tank starts at 10 mL/day. Dilute it into your top-off water or split across the day with a dosing pump — never dose a slug of straight vinegar into a single point.

Why so low? At the starting dose the bacterial population grows slowly. Slow growth means low cloudiness risk and easier observation of effects. The first 4–6 weeks at 0.1 mL/gal you might not see any nitrate or phosphate change — the bacteria are just settling in. After that, the export rate compounds.

Ramping up

Test nitrate and phosphate weekly. If both are still above target after 4–6 weeks (NO₃ > 5 ppm, PO₄ > 0.10 ppm), bump the dose by 10% per week. Don't go faster. Each step is roughly 6–8 weeks behind the previous step in terms of observed effect because the bacterial population takes time to grow into the new carbon supply.

Realistic upper bound: most tanks stabilise between 0.5 and 1.5 mL of vinegar per gallon per day. Heavily-fed SPS tanks with high biomass can run up to 2 mL/gal. Anything beyond that and you're asking for trouble — see the side effects section.

If you switch from the starting dose to a maintenance level and nitrate or phosphate climbs again, that's normal — re-ramp slowly from where you are.

Vinegar vs vodka vs sugar

All three are discussed at length by Randy. The differences:

• Vinegar (5% acetic acid): 0.05 g of carbon per mL of vinegar. Slowest, gentlest, lowest cloudiness risk. The default for modern protocols. Long shelf life. Costs a few dollars per gallon at the grocery store.
• Vodka (40% ethanol): 0.16 g of carbon per mL of vodka — roughly 3× the carbon density of vinegar. Faster results, higher cloudiness and cyano risk. Mainly historical; most reefers have moved to vinegar.
• Sugar (sucrose, dissolved in RO water): Highest carbon density but also the most likely to trigger explosive bacterial bloom. Almost never used as the primary carbon source today.
• Combination dosing: Some reefers (and the Triton method) run a blend of vinegar plus vodka to combine vinegar's gentleness with vodka's speed. Works fine if you ramp slowly.
• Solid carbon (NP biopellets): A polymer in a reactor — bacteria grow on the surface and slough off. Different operationally but the same chemistry.

Side effects

Carbon dosing is generally safe but the failure modes are real:

• Cloudy water (bacterial bloom). Your dose grew faster than your skimmer can export. Drop the dose by 50% immediately. Cloudiness usually clears within 48–72 hours.
• Cyanobacteria on the rocks or sand. Cyano outcompetes the desired heterotrophic bacteria when conditions are too phosphate-limited. Counterintuitively, the fix is often to back off the carbon dose, not push harder.
• Sudden alkalinity drop. Bacterial respiration consumes alkalinity. Reefers running heavy carbon dosing typically see 0.5–1 dKH/day consumption above coral demand. Compensate via two-part or AFR.
• Coral paling. If you push nitrate below 1 ppm or phosphate below 0.02 ppm, SPS coral may pale or lose colour. Back off the dose; let nutrients recover.
• Cyanobacterial mats and dinoflagellates. In ultra-low-nutrient conditions, these are the organisms that can survive when the desired bacteria can't. Their appearance is the warning sign that you've over-dosed.

When to stop

Stop when nitrate sits stable in the 2–10 ppm band and phosphate in 0.03–0.10 ppm. You don't need zero — coral wants nutrients, just not too many. The reef-tank ideal is "measurably low," not "undetectable."

If you decide to stop entirely, ramp down over 4–6 weeks. The bacterial population will starve and die off; the dead biomass releases nutrients back. Going from 1 mL/gal/day to zero overnight can spike nitrate visibly within a week.

Cross-reference with the nitrate reference, the phosphate reference, and the nitrogen cycle guide for the supporting chemistry.