Carbonic Acid

Carbonic acid forms when carbon dioxide reacts with water. It’s in almost every body of water, and it’s part of what gives carbonated drinks their fizz.

In a reef tank, carbonic acid is why extra carbon dioxide pulls pH down.

Why It’s Important

• Respiration: carbon dioxide from aerobic respiration can slightly lower pH by forming carbonic acid.
• Lights off: without photosynthesis at night, carbon dioxide builds up and pH can drop.
• Nitrate removal: denitrifying bacteria give off carbon dioxide too. The Guide recommends dKH 7+ and pH 8.2+ while removing nitrate this way.

How It Works

Alkalinity buffers against it. The Guide’s target pH of 8.3–8.5 sits near the point where the carbonate/bicarbonate buffer works best against swings.

For low pH, the Guide recommends fresh oxygenated air and adding kalkwasser to the routine. Bio-Kalkwasser™ turns excess carbon dioxide into calcium carbonate, which raises pH, and it’s made to counter the nightly drop.

Carbonic Acid and Related Terms

Carbonic acid is the link between CO2 accumulation and a falling pH. The carbonate system and alkalinity act as the buffer that holds pH stable, and kalkwasser is one way to push back.

The Bottom Line

Carbonic acid forms from carbon dioxide in the water, and it’s behind the most common pH dips in a reef. Steady alkalinity and good gas exchange keep it in check. More on nitrate removal in Nitrates in Saltwater Aquariums.