As with any other check valves, this will prevent water from working its way into your air pump in case of of a power outage or during water changes. The valves come in a clear plastic bag with no instructions, but you can simply blow into the valve and whatever end the air comes out of, that's the direction to install them. If you look at the valve closely, you'll notice the inner part, a white piece of plastic, the narrowest part of it should be pointing towards your aquarium; Quick tip, you'll also notice the narrowest part has engravings around it that look like ^ ^.
Installation is easy, but if you have standard size hosing, do not expect it to slide all the way into the valve, because the tip gets wider as it reaches the valve's chamber. I installed the tube as far as I could and then gave it a good tuck to ensure it wouldn't come off during operation, it stayed nice and tight with no air leaks.
All in all, a good, inexpensive way to protect your air pump. A must have. Hope this helps.
P.S. I took and edited a couple of pictures of these. You can view them in the item gallery/images in case you are wondering the direction of which to install these.
UPDATE (Jan.2013):
It's been almost a year since I bought these (Jan.2012) and they still work. I have two air stones in my aquarium running 24/7 and the valves still let air through constantly and without a problem.
These work and look nicer than the cheaper blue ones. What more is there to say about a little plastic valve. In response to the reviewer that couldn't figure out how they work. When you blow into the hole on each end, one way will allow air to pass through and the other won't. Install the tubing from the pump so that it can blow the air through the valve.
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They are easy the put in the plastic tubing you use for the air supply. Nothing really to exciting to write about. They work.
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It takes a lot of pressure to push air through these valves. I couldn't even blow through one of them when I got it. I hooked them up to my shop air compressor and this seemed to loosen them up a little but I would not recommend them.
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They work as intended, but require a verry strong pump or it will simply block air both ways. Not Great, meh at best.
Keeps my wood floors nice and dry. I have spilt alot of water on this thing and nothing leaks through. Def a must for any fish tank owner.
