Since the valves are on the way, I thought I'd lay out a mini project plan.
First off, I'd like to do a near replacement for the commercial unit: valve, timer circuit, and a control knob, and I'd like to do it as a very simple, firmware free timer circuit that nearly anyone can build using readily available parts in a through-hole soldering project. With the cost of the valve + shipping, I'd hope you could piece together the whole enchilada from various sources for less than $60, and all you need to do is own a soldering iron and have basic soldering skills.
I'd also like to do a fully integrated pH controller version, but I'll talk about that later.
(I'm going to replicate the build instructions to a single blog post - since ReefCentral doesn't let you edit posts, the following will be an up-to-date reference on any part changes, build instructions, etc.
https://theatr.us/elec/simple-carbon-doser/
)
The Clippard "Mouse" valves are a very quiet, very fast acting solenoid. The goal is to pulse on the solenoid for a fraction of a second on a regular basis (every half to 10 seconds, or thereabout).
I'm basing the build on the 12V EV normally-closed standard valve:
http://www.clippard.com/part/EV-2-12-H
I'm planning on using the venerable 555 timer chip, which has existed for nearly forever, but is a perfect quick way to make a variable low frequency timer and is widely available from nearly any electronics outlet. The 555 timer will be controlled by a potentiometer, letting you twist a knob to adjust the speed at which the valve opens. A second potentiometer could be added in order to control the valve open time, or it can be fixed.