Guide · Tubing and check valves
Which way does a check valve go?
How to tell which way round an aquarium check valve fits (arrow or flapper side), plus where to put it and why a standard valve is not for CO2.
Short answer
Air must flow from the pump, through the valve, to the tank. On a valve with an arrow, the arrow points toward the tank: Tetra says its CV4's arrow points in the direction of air flow.[1] Aquarium Co-Op's valve has no arrow; its flapper end faces the air pump.[2] If no air comes out after you fit one, it is probably backwards.
How to read the marking
A check valve lets air through one way and blocks water coming back the other way. Each maker marks the direction its own way, so read the valve you have:
| Valve | Marking | Fit it so |
|---|---|---|
| Tetra CV4 | Arrow | The arrow points the way air flows, toward the tank[1] |
| Danner check valves | Flow direction marked on the valve | Air flows in the marked direction[3] |
| Aquarium Co-Op check valve | Flapper end | The flapper end faces the air pump[2] |
If your valve has no marking you can read, blow gently through it before fitting: air should pass one way only. Fit it so the open direction runs from pump to tank.
The quick test after fitting
Turn the pump on. If bubbles come out of the air stone, the valve is the right way round. Aquarium Co-Op says that if its valve is installed backwards, no air flows.[2] A backwards valve also makes the pump push against a closed line, so fix it straight away.
Where to put it
Direction is half the job; position is the other half.
- Outside the water. Danner says its valves go in the airline outside the water, to stop water siphoning back into the pump in a power outage.[3]
- Near the top of the tank. Aquarium Co-Op recommends placing the valve near the top of the aquarium, so water is stopped at the rim rather than down near the pump.[2]
The pump placement guide covers when you need a valve at all.
Match the tubing size
Valves are made for a tubing size. Tetra's CV4 fits hose with a 4 mm internal diameter,[1] while Danner says its valves fit most common 3/16 in inside-diameter airline.[3] Those are close but not the same size; see 3/16 in vs 4 mm airline tubing.
Not for CO2
Aquarium Co-Op says its standard check valves are for air pumps only and not designed for CO2 systems; it sells a separate stainless-steel valve for CO2.[2] On a CO2 line, use a CO2 check valve that its maker rates for CO2.
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Questions
What happens if I install a check valve backwards?
No air gets through. Aquarium Co-Op says that if its valve is installed backwards, no air flows, which is the quickest sign you need to flip it.[2]
Does the arrow on a check valve point to the tank or the pump?
Toward the tank. Tetra says the arrow on its CV4 points in the direction of air flow, which runs from the pump to the tank.[1]
Can I use a regular check valve on a CO2 line?
Not Aquarium Co-Op's: it says its standard check valves are for air pumps only and not designed for CO2 systems. Use a valve its maker rates for CO2.[2]