An airstone turns air from a pump into bubbles that churn the water. That churn helps oxygen dissolve in and carbon dioxide escape, and it moves water around the tank. It doesn’t filter anything. You want one in a crowded or warm tank, and you usually don’t in a tank injecting CO2.
What does an airstone do? It breaks the air from an air pump into a stream of bubbles, and those bubbles stir the water and its surface. That stirring helps oxygen dissolve and carbon dioxide leave, so fish get more of the oxygen they need. The bubbles themselves matter less than the movement they cause, and a stone won’t clean water or replace a filter.
This explainer sits in our Air Pump guides. To use one to boost oxygen, follow how to add oxygen to a fish tank.
What Does an Airstone Do?
An airstone is a small weighted block, often porous, that connects by airline tubing to an air pump outside the tank. Air pushed through it comes out as fine bubbles. Aquarium Co-Op describes it as a way to raise dissolved oxygen, which should ideally sit around 7 to 8 mg/L in a freshwater tank.
| It does | It does not |
|---|---|
| Stir the water and its surface | Filter the water |
| Help oxygen dissolve and carbon dioxide escape | Remove ammonia or nitrite |
| Move water around the tank | Replace water changes |
| Give an air-driven sponge filter its power | Make an overstocked tank safe |
Do the Bubbles Add the Oxygen?
Not quite. Aquarium Co-Op says the popular belief is a myth, and the key is agitation at the water surface, where carbon dioxide leaves and oxygen enters. Aquarium Science says bubbles help too, as long as the pump is big enough to make hundreds a second. The sources differ on the mechanism, and agree on the result: a churning surface means better oxygen exchange.
When You Need an Airstone
- Fish gasping at the surface, resting on the bottom or eating poorly, which are Aquarium Co-Op’s signs of low oxygen.
- A warm or crowded tank. Warm water holds less oxygen and more fish use more of it.
- A tank with weak surface movement, such as one with a sponge filter or a filter outlet that sits underwater.
- Power-cut backup. A battery air pump and stone keeps water moving.
When you may not want one: in a tank injecting CO2 for plants. The bubbling can drive off the dissolved CO2 your plants need. Also check that each fish species you keep is comfortable with the current, since some prefer gentle water. Needs vary, so check yours.
How to Use an Airstone
- Connect the stone to an air pump with airline tubing.
- Place the stone near the bottom of the tank, in the open, not buried in gravel.
- Fit a check valve in the line if the pump sits below the water level, so water can’t siphon back into it.
- Match the pump to the tank. A pump that is too small gives a trickle, and one too big is noisy and wasteful.
- Clean or replace the stone when the bubbles thin out, because clogged stones lose effect.
The check valve advice matters most when the pump is below the tank, and water can flow back down the airline and damage it. Placement, noise and pump size are covered in more depth in our setup guide.
Compare gear in our air pump and air stone picks, or use an air-driven sponge filter that gives filtration and aeration together.
Bubbles Explained
| What you see | What it usually is | Action |
|---|---|---|
| A stream of bubbles from a stone | Normal aeration | None |
| Tiny bubbles on the glass after a water change | Micro-bubbles from cold tap water | None; they clear overnight |
| Bubbles that stay clustered on the surface | Several possible causes | Test the water and see the bubbles guides |
For unexplained surface bubbles and foam, see
Related Guides
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Frequently Asked Questions
What does an airstone do in a fish tank?
It creates bubbles that stir the water, helping oxygen dissolve and carbon dioxide escape. It does not filter the water.
Where should the air stone go in a fish tank?
Near the bottom, out in open water rather than buried in gravel, with the pump outside the tank.
Do I need an air stone if I have a filter?
Not always. If your filter outlet ripples the surface and fish look comfortable, you may not need one. A crowded, warm or low-flow tank benefits from extra aeration.
Can an air stone be on 24 hours a day?
Generally yes. The exception is a tank injecting CO2, where the bubbling can drive off the CO2 your plants use.
How often should you replace an air stone?
When the bubbles thin out or the stone clogs. No fixed interval came up in our sources, so judge by the flow.
