I’ve been using a dosing system like the one prafferty posted a diagram of for about a month now, and so far so good. For something so simple, it took me forever to piece it together. For what it’s worth, here are some things I learned by research and trial and error:

* The container should be rigid. I use a six-gallon carboy I bought at a local home brewing supply for about $20.

* Two-hole stoppers can be found at Analytical Scientific.

* I use rigid plastic tubing I got at Petco to go through the stopper into the container. I originally used glass tubing, but it is hard to keep it from slamming against the side of the container when removing the stopper to refill, which was unnerving, and I eventually broke it by stepping on it while refilling.

* For the siphon stop (the B and C part of the diagram), I use 2” PVC and an endcap with a hole drilled through it just big enough to squeeze in the air-line tubing connects it to the container. Then I siliconed around the tubing to make it air-tight.

* Make sure the siphon stop is large enough to stop the siphon. I’m using 2-inch PVC that extends about 10 inches above the sump water surface, and I still get water in the line which messes things up. I did a quick fix for this by removing the sag in this line so it drains.

* For a long time I couldn’t figure out how to mount the siphon stop in the sump. Finally I figured I could just make it long enough so the open end of it could rest on the bottom of the sump and have a hole drilled into the side of it where I want the water level to be. It is not secured at all; it just rests on the bottom and leans on the side of the sump. Seems kind of lackadaisical, but it works and I don’t know how it could fail. I guess the drilled hole could clog, but you can make the hole as big as you want.

* Keep the outlet (part that drips) above the water or it will clog, but close to the sump water surface to reduce air contact.