My DIY Skimmer

The image to the right is the skimmer as I originally built it. If you click on the image there is a diagram of the skimmer parts. The skimmer has changed a little bit due to better design considerations and I will elaborate on that in the Diagram pop-up.

There will also be a short dissertation on dealing with acrylics as this was the first item that I constructed anything using the various glues and techniques. A word of warning, if I had it to do over again I would have learned about the techniques used with acrylics by guinea-pigging an easier project. The skimmer is actually a fairly complicated device to manufacture even though the concept is easy.



Click Image for Diagram and Other Images

Initially the skimmer was designed with a 4" main chamber and 5" top and bottom plates manufactured from 1/8" acrylite. This proved to be too wimpy and the bottom plate had to be doubled in thickness. The colletion cup was a fixed design, meaning that the skimmer had to be removed from the tank to clean the cup. This became a rather tiresome and unpleasant job due to the fact that the aquarium cabinet being used was only an inch taller than the skimmer, hence I had to cut a hatch in the cabinet top to allow the skimmer to be removed and replaced. Bad design from the beginning. The following are the modifications that the skimmer has undergone since its initial inception and use.

Skimmer Modifications

After using the skimmer installed as an external device for a few weeks, it became clear that the bottom plate, as previously mentioned, was not strong enough, and that the collection cup design was flawed. It was with much trepidation that I undertook the modification of these flaws. Fortunately the aquarium itself was still in the first months of cycling and settling in so there was no real need for the skimmer. I was able to remove the skimmer with no detrimental effects to the tank and its inhabitants.

First, it was necessary to remove the bottom plates form the main chamber and the reaction tube. Daunting as it seemed this proved to be relatively simple. After dismantling the unit I used a router with a 3/8" ballbearing bit to cut off the flange of the plate on the main chamber. This allowed me to use a chop saw to cut the bottom off straight.

Second, I again used the router to remove the outer part of the plate attached to the reaction chamber and then again cut off the end with the chop saw.

Thirdly, it was necessary to manufacture two new plates. I used 1/4" acrylic to insure the seating of the gasket properly between the two plates. I cut and shaped these two plates at the same time to make sure they would bolt together precisely. Before reassembly, I needed to cut off the collection cup leaving enough of the tube coming out of the bottom of the cup and up out of the main chamber to be able to reattach them in some fashion. Then it was necessary to reattach the plates to the main chamber and the reaction chamber so that the reaction chamber would line up with the air/water input tube and that the holes in the plates would line up properly. I also did some modification to the output fitting that allowed me to attach a gate valve directly to the fitting.



(I recommend highly using a gate valve as it is much easier to adjust than a simple ball valve.)

Then lastly, I had to figure out a way to attach the collection cup to the top of the skimmer so that it was somewhat water tight and easily removed through the hatch in the top of my cabinet. This was a process of calling everywhere trying everything and racking my brains to find some sort of fitting that would accomplish just such a task.

Commercial skimmers have several different methods to solve these problems. The types of fittings used are not readily available to the average DIY guy, 2" acrylic tubing does not have an equivalent sizing in PVC that is appropriate. The answer, of course, was right in front of me. I noticed that the size of the end piece on my siphon/gravel vacuum device was the same size as the 2" acrylic. This meant that the top of the gravel filter part was a cap for the 2" tubing. I drilled some holes in the top of the fitting and cut it out with a coping saw and then filed and sanded it until it was an effective coupling for the 2" acrylic on the bottom of the cup and the top of the skimmer body. So far it has worked beautifully.

Adjusting the Skimmer

Let me preface this article by saying that the technical information included here is informative only for the venturi type skimmer but the visual adjustment for any skimmer is basically the same. The next learning curve I now needed to get past was how to adjust the skimmer to its optimum efficiency.

When I first built the device I really did not know how big of a pump that I needed. Through the internet, and also asking questions on several different forums, I gleaned that the GPH at the output of the skimmer should be 2 times the volume of the tank per day...how could this be?..I had been told that the GPH for the tank should be somewhere between 10 and 15 times the volume of the tank. Fortunately, I was able to find some links that explained this information and also that this information was not quite accurate. The flow into the skimmer is different than the flow out of the skimmer because you are injecting air into the water that is going into the skimmer body therefore the volume of water/air is less than the volume of water (hopefully devoid of the air bubbles now) coming out of the skimmer.

At first this is very confusing, at least to me it was. If you have a 100gal tank the output of the pump should be at least 200gal per hour. When injected with air this translates into about a 50% reduction of water flow. In the end because you want to force air through the water not the water through the skimmer (an argument that is still going on around the world) the result is about 75% of the total volume of water should pass through the skimmer twice a day, not twice the volume of the tank through the skimmer per hour. Remember that what you are trying to accomplish is to remove debris from the water by floating bubbles of air through the water so the longer the air bubbles are in the water and in contact with the debris the more of it is picked up by the air and subsequently taken up through the chamber and into the collection cup. If you are moving water through at a very high rate the skimmer loses its functionality in that all you are doing is pumping water through a tube.

All of this was imparted to me in a confusing manner, at first, because the people I questioned as to how a skimmer worked continued to tell me that the way that a skimmer worked was simple. I do not think that it is that simple when it comes to designing and implementing the designs function.

Choosing the correct pump is not such a simple matter either. The pump must be able to handle the back pressure that you are going to subject it to. The back pressure is what allows the water to rise to the top of the main body of the skimmer, which in this case is a mear 24 inches, and stay there creating the lovely foam that we want to bubble up into the cup. Now if you take into consideration that this is 24" x 4" in diameter you must restrict the flow of water out of the skimmer by a considerable amount to allow the water to rise to the top of the body. This is why it is important to have an adequate pump and a gate valve on the output. At first I tried an Via Aqua 1300 (300+ GPH...not enough pump..it would fill up the chamber but at the expense of the output at the gate valve was about 20gal a day. I am now using a Quiet One 3000 (800GPH). This pump has proved to be just about right for my needs.

Small commercial skimmers are usually set up to use powerhead pumps (RIOs or Maxi-Jets). I do not have much experience with commercial skimmers but I would imagine that for large tanks (200gal and up) that they would be well worth the investment. For smaller tanks the market is flooded with options ranging from $120 to well over $300. The skimmer I am using would cost about $75 to make and the pump cost me $40. I think it probably works as well if not better than anything out there for the smaller home reef tank.

Initially,with the ari valve for the venturi closed, establish a good flow of water through the skimmer so that the water level is at the top of the main chamber just below the collection tube. Next open the air valve for the venturi enough that a stream of bubbles with a milky texture is visible. Adjust the output gate valve so that the water and air bubbles go up the collection tube and stops an inch or so below where it would flow into the cup. Take into consideration that depending on the type and size of the skimmer these adjustments may take a while to become stable. Be patient and allow the level to stabilize, adjusting accordingly until the water/bubble level remains the same. Run the skimmer at this adjustment for at least a day. Again adjust the output so that the level drops to just at the top of the main chamber, wait for the level to stabilize, and check periodically to make sure that the level is maintaining. The skimmer will be foaming but probably not producing any skimate. If this continues for more than 3-4 days it merely means that there is not anything to skim. Leave it running and check often. When the skimate does appear, empty the collection cup as needed or at least one a week and clean the collection tube making sure that any sludge etc. does not go back into the tank. That should start it off. Try not to adjust the skimmer any unless absolutely necessary.

Pros, Cons and New Ideas

There are several drawbacks to attempting this project. It is almost necessary to have a table saw, drill press and router to work with acrylic and these tools do need some specific bits and blades. So for the DIY person that does not have a shop already set up it is usually beyond the capabilities of most home shop setups. You could use hand tools however you would have to have ultimate patience and some very good files, drill bits and saws to accomplish the same result. Finding a good plastic source can be a problem if you are living in rural areas. The internet has helped in many ways but the cost of shipping and the limitations on quantities and sizes of pieces imposed by the manufacturers makes some of these materials rather pricey. If you are ready to take on more than one project it can work to your benefit if you go in on the cost with someone else and make work on several projects at the same time.

I should also mention that the top was a bit of a design bugaboo. The original plans called for a 4" vinyl circle as the top of the main chamber and that the collection cup tube was to be 2" and centered on the vinyl circle. The problem I ran into was that the 1/2" PVC pipe serving as the input was very hard to place between the walls of the outer chamber and the collection cup tube. This would have been much easier with a larger main chamber, say 5-5 inches allowing for more room overall on the top piece.

Enlarging the main chamber also would not hurt the overall design as it would allow more time for the bubbles to float back up to the top, although with the present design I have no problems with the bubbles being ejected back into the tank water column. The input tube itself was somewhat of a nightmare because of the size problem discussed above. I had to use a spring to bend a piece of 5/8" acrylic tubing to curve the input pipe. This piece has now broken (twice I might add) and now I have filed off the sidewall of a 90 degree 1/2" PVC fitting to allow it to fit between the outside wall of the collection tube and the input tube glued into the top.

Currently, I am using my first design with the aforementioned modifications. The second design is already about 1/3 finished and will incorporate a three pass reaction chamber. The inlet tube (coming up from the bottom), a second chamber (coming down from the top) that will also serve to switch the bubble flow to a cross-current configuration, and the outer chamber. This is the another big drawback. Once you design one (if you are the intrepid DIY person) you will think of 50 things that you could do better. The good part about this is that it is very hard to fix design flaws in commercially available skimmers, not so with the DIY project. Somehow I feel that there will be at least a few more design improvements coming up in the future.



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