Tetra Air Dome Diffuser Retrofit



Kevin McGuire
kmcguire@morgantechpartners.com

Introduction

If you are like me, even after a great deal of study prior to building your Koi pond, there are a number of things you would do differently now-ok, in my case, there are hundreds. I had always planned on running air stones in my pond powered by an air pump (Sweetwater Model SL22.) This way I could always rely on my DO (dissolved oxygen) levels being near maximum. However, in my plans I failed to appreciate that I could also improve my pond drain's ability to clean at the same time-a twofer if you will. After seeing other ponds on the internet and observing message threads on the internet BBS's (bulletin boards), I decided to dump the 3" air stones in favor of an air diffuser retrofitted to the dome of my Tetra 3" bottom drain.

The air diffuser I used is sold by Aquatic Eco-Systems (AES.) It uses a rubber EPDM membrane punctured by many fine holes that once pressurized by air will expand and shed fine to medium sized bubbles. This EPDM membrane is totally clog-free. As opposed to air stones which will, at some time or another, clog. Consequently air stones ultimately need to be replaced or cleaned. The use of air diffusers affixed to bottom drains is a relatively recent improvement in Koi pond design. I've read that this technology was originally developed by the petroleum industry for keeping the under sea pipelines free from debris. In a Koi pond, the concept is that the rising air bubbles will create a low pressure area above the EPDM membrane. Water and debris will flow or rush into this low pressure area which is located on the bottom of the pond directly above the bottom drain(s). The bottom drain is therefore assisted in pulling out the negatively buoyant (heavier than water) debris. I have no data as to their efficacy for neutrally buoyant (suspended in the water) debris but I tend to believe that there is some help there as well. Positively buoyant (floating) debris is not likely to be removed by this mechanism; you must design in a skimmer in your Koi pond for positively buoyant matter.

Adding air diffusers to bottom drains has become a popular retrofit. I decided to snap pictures with my digital camera along the way and post them in this article so that others can see how I did it. Hopefully this article will give you some ideas even if you have another make of bottom drain. If you are still in the planning stages for your pond, by all means include purpose built air dome diffuser drains from the start. There are many excellent 4" diffuser drains out there besides the 3" air dome model that Tetra introduced. In any case, it turns out that my retrofit can be done to the standard 3" Tetra bottom drain for less than the cost of the diffuser model. The DIY design has the additional advantage of not blocking debris from entering the underside of the dome into the drain, something that can be a problem with the Tetra diffuser model.

DIY Difficulty and Materials

This turned out to be a relatively pain free DIY project that required a total of about two to three hours of effort. The difficulty level is low.

Make sure you have all the materials and tools you need BEFORE starting this project.

Materials used:
· Dome to Tetra 3" bottom drain
· Aquatic Eco Systems (AES) Part number AD11 Dome Style Diffuser
· Oatey Epoxy Putty or equivalent from a home store
· Male Adapter ¾" NPT x 3/8" barb - use whichever size barb your airline requires, I use 3/8" weighted airline from AES.
· PVC Elbow ¾" 90° Ell slip x FNPT (one end slip, one end female thread)
· PVC Female Adapter ¾" slip x FNPT (one end slip, one end female thread)
· PVC pipe 1' of ¾" schedule 40 (you won't need it all)
· PVC Cement
· Flat black spray paint (this is optional, the dome will eventually darken over from algae but, what the heck, it looks really sweet from the start if you initially paint it)


Tools used:
· Drill (I prefer a cordless but any type will do)
· 1 ¼" hole saw bit (I used a Wally World cheapo set)
· Dremel tool with a grinding bit (other means could be used but I really like using my Dremel tool. I have the flexible wand attachment that allows me to use the grinding bit like a dentist's drill. Guys and girls if you don't have one yet here is a good reason to get one.)

Construction

First I unscrewed the white collar ring off of the AES diffuser. There might be some grease of the EPDM membrane and underneath the white collar ring, on mine there was. Clean the grease off with some Windex and a clean rag. I have read that this grease will tend to turn white once in the pond. I spray painted the white ring ONLY and left it to dry-the EPDM membrane is already black.

Next, I retrieved my Tetra dome from the bottom of the pond. If you have any small fish it would be a good idea to temporarily shut off the valve to the bottom drain(s). (Not that I've ever had a 6" fish sucked down my drain while the dome was off, hmmm.) I shut off my drain and just ran my skimmer while I did this project.

Carefully pry open the bottom plug from the dome. I used a common screw driver. My plug fits securely so I have never found it necessary to cement it on. If you have cemented yours on you will need to use your hole saw bit to open up a new hole. Patch it with epoxy putty when you are through or construct a new plug. Empty out whatever you used to weigh the dome down. (I use pea gravel and fill the rest of the void space with water.) Then take the garden hose and a sponge and clean off the grunge from the dome. Try to rinse out as much junk from the inside of the dome as you can. If you don't get it all don't worry as you are going to be putting more holes in it shortly. Dry off the dome--I used my air compressor to dry it off, a car shammy works great too.

Click on an image for more detail and larger version

Next take the 1 ¼" hole saw and cut a hole directly on top of the Tetra dome. You should be able to use the injection mold marks as your guide. To do this I placed the dome on the garage floor and held it steady with my knees. I slowly drilled the hole. The hole saw bit will heat up the plastic and melt it as it cuts, this causes it to give off some fumes so try to avoid breathing this stuff. When you are done it is quite likely that the plastic will have some burrs on it. I used my Dremel tool to deburr the hole, a small round file would work nicely too.

We need to modify the hole we just made to enable the 90° PVC elbow to be rotated inside of the dome. To do this I screwed my 3/8" male adapter into the threaded side of the elbow. The barb will act as a temporary handle. Plus, with the barb threaded into the elbow it is too long to allow the elbow to accidentally slip into the dome. I used my Dremel tool to widen one side of the hole. It is best to remove a little of the dome's plastic, then try to rotate the elbow into the hole, remove a little more, try a fit, and keep repeating until the elbow finally slides into the dome. There will be about an eighth to a sixteenth of an inch of play from the dome bottom to the bottom of the elbow when the elbow is flush to the top of the dome.

There may be some ridges from the injection molding process inside the dome on the floor. There was on mine. These ridges will interfere with the elbow fitting perfectly level and flush to the top of the dome. I carefully used the Dremel tool to grind down these ridges flat so that nothing was touching the elbow as it sat flush to the top.

Click on an image for more detail and larger version

Next I located where I wanted the PVC pipe to emerge from the dome. There are three legs that support the dome (a right triangle.) I thought about this a lot and decided to use one of the three dome support legs as a guide to determine the location of the stem exit hole. It seemed to be the most stable place rather than locating the hole between two of the bottom legs. Next I used the hole saw bit to make a hole about ½" from the base of the dome. I put the hole saw bit at about a 30° angle so that I could saw perpendicular to the dome. Be careful not to cut through the bottom of the dome. In my picture below it is hard to see the blue marks I made to indicate where I wanted to locate the hole.

The ¾" length of straight PVC will not be able to fit through this hole and be parallel to the bottom of the dome. I had to use my Dremel tool and grinding bit to make the circle into an oval. This process was entirely iterative and is the longest part of this project. I repeatedly made a test fit, ground some more, made another test fit, etc… When you get done you want the ¾" PVC to slip easily into the slip end of the 90° elbow. This took a lot more grinding than I first thought it was going to take. When I was done I had enlarged the hole nearly to the bottom of the drain and extended it upwards from the top of the initial hole a good way. The barb has a hexagon in it to enable it to be tightened with a wrench. I used one of these hexagon corners to indicate the center line of the 90° elbow while I was test fitting the PVC pipe. You might also want to sand down the exterior of the end of the PVC pipe that will slip into the elbow, this makes test fitting easier.

Again, there may be some ridges from the injection molding process inside the dome on the plastic floor. If these ridges interfere at all with the straight piece of PVC fitting into the elbow freely or the top of the elbow being perfectly level and flush with the dome, get rid of them.

When you are done you want to make sure that the top of the elbow is nice and level to the dome. The whole PVC assembly should have little play in it and not constricted due to rubbing of any sort.

Cut down the PVC pipe so that the edge of the female adapter is a half inch from the side of the dome. Sorry I did not make exact measurements on this length. Cement the female adapter to the pipe.

Click on an image for more detail and larger version

Next comes the artistic part but a word of advice, take off any rings, watches, or bracelets prior to this next step. · Read the manufacture's instructions on how to mix and work with the Oatey (or equivalent) epoxy putty. · Make sure to work with only small amounts of the epoxy putty at a time. · Make sure it is thoroughly blended together before applying it. The Oatey epoxy should be worked until it is uniformly grey. · Once you start to feel the epoxy putty go off (harden up) stop working with that batch. You can always cut off another small piece and mix it up. Working with the epoxy putty that has started to harden will result in a poor or no bond.

Be forewarned, I worked quickly during this next step. Cut off and work an amount of the epoxy putty up-you want some left over for the step in the next paragraph. Take a small ball and press it underneath the elbow. Rotate the elbow into the dome. Squish the elbow down until the top of the elbow is flush and even with the top of the dome, the putty will stick to the bottom of the dome and elbow and allow you to make adjustments. Quickly apply some PVC cement to the end of the stem and install it through the side of the dome and into the elbow. Use the hexagon corner on the barb to show you the center line of the elbow. Work quickly before the cement sets up or the putty hardens.

You should have some left over putty to keep working. Making sure that the top of the elbow stays flush and EVEN with the dome, apply two balls of putty on each side of the elbow in where the top hole was enlarged. Work the putty underneath of the dome with your finger or some other tool like a screw driver or pencil. You want contact with the dome and the side of the elbow. Really pinch it in good and smooth it to a fine taper with your finger. Note: make sure to keep the putty OUT OF THE THREADS-enough said? Quickly take another small ball of putty and work it underneath and to the sides of the ¾" PVC pipe where it exits the dome. Again smooth it to a fine taper with your finger. If you have enough time, apply the rest of the putty (while it lasts and has not hardened) to other sections. Keep in mind you want to squish the putty into the dome to get the best seal. At a minimum, try to get the two sides of the elbow puttied as well as the underneath and sides of the stem where it exits the dome. Keep the top of the elbow flush and even!

Let the putty set for a few minutes and then mix up another small batch and have at it again. I found I could roll up a small cigar shaped piece and cover a long stretch at one time. Always maintain a nice tight taper to the putty that way you can go over it a few times and build it up.

The entire assembly should now be installed in the dome and cemented together. Wait a few minutes and then remove the barb from the top of the PVC elbow. Reinstall the barb to the threaded end of the pipe. You may want to put a couple of wraps of Teflon tape on the threaded end of the barb prior to installing it. I didn't find this necessary. Hold on to the female threaded end of the stem and use an adjustable jaw wrench on the barb's hexagon to tighten it into place. Do NOT hold on to the pipe or the dome to screw in the barb, this may weaken your PVC cement bonds. It should not take very much force to do this.

Click on an image for more detail and larger version

I then used some flat black spray paint to cover up the white PVC and the grey epoxy putty.

Wait until you are sure that the epoxy putty has totally cured. It shouldn't take very long, maybe ten or twenty minutes.

The last step is to screw the AD11 diffuser into the ¾" female thread of the elbow in the dome. You could use a couple of wraps of Teflon tape on the threads if you like. Don't over tighten it. Rotate the diffuser until it is nice and snug and parallel to the ground.

Prior to installing the dome back in your pond, fill up the void space inside of the dome with pea gravel or sand, fill up the remaining void space with water and reinsert the plug. Make sure that the plug is nice and tight.

Click on an image for more detail and larger version

Conclusion

After installing the air diffuser dome in my pond I noticed a dramatic increase in the debris and solids removed and trapped from my pond. Initially the Koi were a little spooked (probably more from seeing my legs than the air dome) but got over it quickly. Now they seem to like lying near the diffuser and swimming through the bubbles. I estimate that I am running about 1.8 cfm of air through the diffuser. No attempt was made to empirically validate this but there seems to be no noticeable drop in my DO levels with the switch from the air stones to the diffuser.

Good luck with your project.
Kevin McGuire

Go back to the DIY menu.