Buying task · Tubing and check valves
Airline tubing and check valves
3/16 in vs 4 mm airline, which way a check valve goes, where to put it so water stops at the rim, and why all airline hardens over time.
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Short answer
Match the tubing size to your pump and valves first: 3/16 in and 4 mm airline are different sizes. Fit a check valve, the right way round, whenever the pump sits below the water level, and put it in the airline outside the water, near the top of the tank. Expect all airline to harden with time and trim or replace it.[1]
What tubing and check valves do
Airline tubing carries air from the pump to an air stone or sponge filter. A check valve is a one-way valve in that line. When the pump stops, for example in a power outage, water can siphon back down the tubing into a pump that sits below the water level. Danner says its check valves belong in the airline outside the water to stop exactly that.[2]
The decisions, in order
1. Tubing size. Danner describes its airline as standard 3/16-inch tubing and says its check valves fit most common 3/16 in I.D. tubing.[2] Fluval lists 3/16 in / 4.78 mm hose for its A Series pumps.[3] Metric parts differ: Tetra's CV4 check valve fits 4 mm internal-diameter hose,[4] and EHEIM lists 4 mm for its air100.[5] The 3/16 in vs 4 mm guide covers fit and materials.
2. Pump position. Aquarium Co-Op says its pump should run above the water level to prevent back-siphoning, and that a check valve must be used if it sits below.[6] See air pump above or below the tank.
3. Valve direction. Each maker marks direction its own way. Danner marks the flow direction on its valves.[2] Tetra says the CV4's arrow points in the direction of air flow.[4] Aquarium Co-Op says its valve goes with the flapper end facing the pump, and that installed backwards, no air flows.[7] Full walk-through: which way does a check valve go.
4. Valve placement. Aquarium Co-Op recommends placing the check valve near the top of the aquarium, so water is stopped at the rim rather than down near the pump.[7]
5. Air or CO2. Aquarium Co-Op says its standard check valves are for air pumps only and not designed for CO2 systems.[7] A planted tank with CO2 needs a valve made for it; see the planted tank page.
6. Material and wear. All airline eventually hardens in water, Aquarium Co-Op says, and cutting off 0.5-1 inch of hardened tubing before reattaching it is its fix.[1] "Never brittle" claims on other tubing do not change that. Aquarium Co-Op calls its black airline food-grade PVC, and the same page carries a California Proposition 65 warning for DEHP.[1] EPA describes DEHP as a phthalate used mostly as a plasticizer.[8] "Food grade" is the maker's term, not a finding about fish.
Every page on tubing and valves
Guides
- Air pump above or below the tank? Back-siphoning and check valves
- Which way does an aquarium check valve go?
- 3/16 in vs 4 mm airline tubing: sizes, materials, hardening
Tank pages
Shortlist. Not yet. A tubing and check-valve shortlist needs at least two products that meet our catalog bar, and we have found no check valve sold by Amazon.com or by its brand that does so far. Buy check valves that state the tubing size they fit and mark their flow direction.
How to choose
Start from your pump's outlet: Fluval's A Series, for example, takes 3/16 in hose.[3] If your pump and valves are 3/16 in, a plain roll of 3/16 in tubing is all you need; buy enough to replace runs as they harden.
Add a check valve that names 3/16 in tubing and shows its flow direction, plus a T connector or gang valve if you split one pump between several outlets.
Key terms
Inside diameter, outside diameter, 3/16 in airline, 4 mm airline, barb fitting and CO2 check valve.
Everything in this topic
Guides
- 3/16 in vs 4 mm airline tubingHelps you buy airline tubing and check valves that fit your pump's outlets, and know when to trim or replace old tubing.
- Air pump above or below the tank?Tells you where to place an air pump and what to add if it has to sit below the tank, so water can't run back into it.
- Which way does a check valve go?Lets you fit a check valve the right way round the first time and place it where it actually protects the pump.
Common questions
Is 3/16 inch airline the same as 4 mm?
No. 3/16 in is about 4.76 mm, and Fluval lists its A Series hose at 3/16 in / 4.78 mm, while Tetra's CV4 valve and EHEIM's air100 use 4 mm hose. More in 3/16 in vs 4 mm airline tubing
Why does airline tubing get hard?
Aquarium Co-Op says all airline tubing eventually hardens when left in water for long periods, and suggests cutting off 0.5-1 inch of hardened tubing before reattaching it. More in 3/16 in vs 4 mm airline tubing
Is 'food grade' airline safer for fish?
'Food grade' is the maker's term; we found no source that tests airline for fish safety. Aquarium Co-Op's food-grade airline also carries a California Prop 65 warning for DEHP. More in 3/16 in vs 4 mm airline tubing
What is DEHP?
EPA describes DEHP as a phthalate most commonly used as a plasticizer. It released its final TSCA risk evaluation for DEHP in December 2025; that evaluation does not address any specific aquarium tube. More in 3/16 in vs 4 mm airline tubing
Is 3/16 inch the inside or the outside diameter?
Danner states 3/16 in inside diameter for the airline its valves fit. Not every listing says which, so check the pump outlet and valve barb before you buy. More in 3/16 in vs 4 mm airline tubing
Can an air pump sit below the aquarium?
Yes, with a check valve. Aquarium Co-Op says its pump should run above the water level, and that a check valve must be used if it sits below; Tetra gives similar advice for Whisper pumps. More in Air pump above or below the tank?
What is back-siphoning?
Water running back down the airline into the pump, usually when the pump stops in a power outage. Danner says its check valves stop that. More in Air pump above or below the tank?
Where should the check valve go on the airline?
Outside the water, and Aquarium Co-Op recommends near the top of the tank so water is stopped at the rim rather than down near the pump. More in Air pump above or below the tank?
Questions
Which way does an aquarium check valve go?
It depends on the valve. Danner and Tetra mark the flow direction; Tetra says the CV4's arrow points in the direction of air flow. Aquarium Co-Op says to fit its valve with the flapper end facing the pump, and that installed backwards no air flows. Details in which way a check valve goes.[2][4][7]
Is 3/16 inch airline the same as 4 mm?
No. 3/16 in is about 4.76 mm, and Fluval lists its A Series hose as 3/16 in / 4.78 mm, while Tetra's CV4 valve and EHEIM's air100 use 4 mm hose. Match the tubing to the pump outlet and valves you have.[3][4][5]
Do I need a check valve if my pump sits above the tank?
Aquarium Co-Op says to run its pump above the water level and to use a check valve if it sits below. Tetra advises a drip loop and an in-line check valve, or placing the pump higher than the tank if that is not possible.[6][9]
Can I use an air 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, and it sells a separate CO2 valve. Check your valve maker's statement.[7]
Why has my airline tubing gone stiff?
Aquarium Co-Op says all airline tubing eventually hardens when left in water for long periods, and suggests cutting off 0.5-1 inch of hardened tubing before reattaching it.[1]