A protein skimmer is used to export unwanted bio-organic wastes. These wastes are attracted to the tiny bubbles produced by the venturi valve. The smaller the bubbles, the more waste is attracted. The bubbles then carry the waste to the top where it is collected and removed from the system. When running properly the foam out of the top of the skimmer should be brown and dry.Jan. 2004 Update: I now feel the proper way to run a skimmer is much wetter than what is described above. It should yield a tea coloured skimmate and not a brown sludge. This removes more bio-organic material.
The new skimmer is a triple pass version. The body is now almost a foot longer than before. I am now able to get the full flow out of my Mag 7. It also seems more efficient now. The foam is dryer than before. I do not get a lot of true skimmate out of my system. I think this is due mainly to my very low bioload and a very large DSB.Jan. 2004 Update: The DSB acts as a sink for bio-organic material this is why I was not getting much skimmate. I have since removed the DSB for this reason.
I think a protein skimmer is the easiest and least expensive piece of equipment to try your hand at DIY aquarium stuff. I would recommend building a skimmer to anybody. Unless you are using a custom made, very expensive skimmer, it is very likely you could make one that would do a better job than the one you are using. Building your own also allows you to fit the skimmer where you want it. A pump is by far the most expensive piece on a skimmer. Just about any pump will work on a skimmer. If it is a little powerhead kind of pump then I would recommend building an airstone driven counter-current design. These skimmers only need water flow from the pump. There will be little back pressure on the pump. Most powerheads do not operate well if there is pressure. Instead of using a venturi valve to supply the air bubbles, an airpump with an airstone will be used. If you have an inline pump then a venturi would be possible. The following design is a triple pass skimmer using a venturi valve. The materials for the skimmer can be found at just about any home improvement store. The materials needed should cost you around $35, not including the pump. Not bad at all.
I decided on a venturi for ease of maintenance. Some people believe the airdriven skimmers can be the most efficient if tuned and maintained properly. The airstones need to be changed fairly often in an airdriven skimmer, which for some people is not a problem, but for me, I am lazy.
The design philosophies for both the venturi and the airdriven skimmer are the same, the only difference is the delivery of the air. You can convert one to the other fairly easilily.
three important design considerations in building a skimmer are: