This is a very simple project that I developed, Eheim products are horribly expensive in Mexico, It was improved by Ruud and now published (in Dutch) in his pages.
All you need is a timer, syringes, 2 check valves, a "T", small screws, pieces of sheet metal, some spare time.
I didnīt use the aluminum Ruud used as a base for the syringe but instead used a piece of PVC tubing (A Dremel tool help me carved it). A small screw works as a pivot (placed where "Rotate" is) I use a piece of sheet metal instead of that strange beige thing
A check valve in one side allows the fertilizer to go into the syringe, another one allows the fertilizer to go to the aquarium
Terumo syringes work better than BD, I tried all sizes from 60 ml to 1 ml, only 1 and 3 ml worked fine, bigger ones stuck, the injection cycle lasts 12 hours
Here's a photo (from Ruud pages)