Why Citric Acid and Baking Soda React to Produce CO2—and Why You Shouldn’t Dump It in Your Tank
The Chemistry: How Citric Acid Produces CO2
When you combine citric acid and sodium bicarbonate (baking soda) with water, you get a vigorous chemical reaction that produces carbon dioxide gas. The mistake many people make is thinking the carbon atoms in the citric acid molecule itself break apart and become CO2. That’s not what happens.
Instead, here’s the actual sequence: citric acid (H3C6H5O7) donates protons (hydrogen ions) to the bicarbonate ions. These protons combine with bicarbonate to form carbonic acid (H2CO3). Carbonic acid is highly unstable in water and immediately falls apart into carbon dioxide gas and water.
The full balanced equation looks like this:
H3C6H5O7(aq) + 3 NaHCO3(aq) → Na3C6H5O7(aq) + 3 H2O(l) + 3 CO2(g)One molecule of citric acid reacts with three molecules of baking soda, producing three molecules of CO2. The CO2 emerges as gas—those bubbles you see in a bath bomb or a DIY CO2 experiment.
Why DIY Aquarium CO2 Systems Use This Reaction
Aquarium plants need CO2 to grow. Pressurized cylinder systems work well but cost hundreds of dollars. A citric acid and baking soda system is much cheaper. The typical setup uses two sealed bottles connected by tubing. One bottle holds citric acid dissolved in water; the other holds baking soda and water. When you connect them, the acid flows into the baking soda bottle, the reaction happens in that sealed space, and the resulting CO2 gas is driven into the aquarium through a diffuser.
This works because the reaction is quick, consistent, and predictable. You can also easily refill both bottles at home, making it a practical alternative to commercial CO2 systems for small to medium tanks.
Why You Can’t Just Dump Citric Acid Into Your Tank
Here’s where the thread question points to a real problem: adding citric acid directly to your aquarium water is a bad idea, even though the chemical reaction itself is harmless.
When citric acid dissolves in water, it’s a strong acid. To generate enough CO2 to meaningfully enrich your tank, you’d need to add a lot of it. This causes a sharp, sudden drop in pH. Fish and plants can’t handle rapid pH swings. A sudden pH crash can stress or kill fish and damage delicate plants. It’s the opposite of the gradual, controlled CO2 enrichment you get from a proper system.
Additionally, if you added both citric acid and baking soda directly to the tank hoping they’d react, you’d get an uncontrolled reaction that either happens too fast or doesn’t happen efficiently at all. The chemistry works best when both chemicals are in a sealed, confined space where the gas can build up and be driven into the aquarium under gentle pressure.
The Role of Carbonic Acid and Buffering
Carbonic acid plays another important role in aquarium chemistry beyond just CO2 delivery. When CO2 dissolves in water, it forms a carbonic acid-bicarbonate buffer system. This buffer helps stabilize pH in the tank and is part of the natural carbon cycle. A proper DIY CO2 system delivers CO2 slowly enough that this buffering system can work smoothly.
Building a Safe Citric Acid CO2 System
If you want to use citric acid for CO2 enrichment, follow the DIY system approach. Start with two 1-liter bottles. Fill one with citric acid (typically 150g) and warm water. Fill the other with baking soda (typically 200g) and water. Connect them with sealed tubing, run a second tube from the baking soda bottle to your aquarium (with a check valve to prevent backflow), and place the diffuser in your tank. The reaction starts within minutes, and you can control the rate by adjusting how much acid flows into the baking soda chamber.
These systems work well for smaller tanks (under 100 liters) and need refilling every 2–3 weeks, depending on your tank size and plant load. They’re reliable, inexpensive, and—when set up properly—completely safe for your fish and plants.