Copied from the Durso website:
The End-Cap is held on with teflon tape. A very small, as small as I could make it (thickness of a toothpick), hole is drilled in the top of the End-Cap fitting. This allows some air to enter into the standpipe. This size air hole worked well for my setup, you will likely need to tinker with it. I would suggest starting with a 1/16 inch drill bit for the air hole in the End-Cap. If you find the water level in the chamber fluctuates quickly then the standpipe needs to suck in more air. Try a 5/64 inch drill bit to make the hole slightly larger -- increasing the size if need be. Do not be surprised if you need to go as large as ¼ inch with the hole size. The lower the flow the larger the hole size needed.
The hole on the top of the standpipe (in the End-Cap) is very important. Without the hole, a full siphon will be created and water will be sucked out of the overflow chamber to fast. The water level will drop below the intake and you will get a terrible air sucking noise. If you drill a hole and the water level still drops to far (making a sucking noise) then the hole is to small. Just make it slightly larger (see above) and the water level will raise. If you make the hole to large then the water level will be to high. (It should not overflow the tank as it will not get that high, but keep an eye on it). If the water level is too high you may need to replace the end-cap (they are cheap). Many people report that it is easier to tune with several smaller holes than a single larger hole.
Ideally the water level in the overflow chamber will be about 1 to 2 inches lower than the display water level. This provides for gas exchange and prevents a slime film from building up on the water surface of the overflow chamber. If you designed it correctly, the water level in the overflow chamber will be a a little lower than the display tank and the water level will be at about the middle of the Street-Ell fitting as pictured in the introduction page. If the water level is slightly higher, no big deal. As long as the overall water level in the chamber is lower than the display tank it is stable that is good enough.
The height of the Durso Standpipe is not critical. I would recommend the height of the end-cap (top of the Durso Standpipe) to be about equal or slightly higher than the top plastic rim of the tank.
