Hey Bob, glad I was able to entertain a little
This is working very well. I have it dosing micros for my 100gal, 36gal, and (later) 55gal tanks. I started it back in August and had enough liquid to dose 4 months, so I checked levels in December, and noticed that the 36gal dosing worked perfectly, while the 100gal was only about 50% of what I had planned. I exchanged one of the checkvalves with a fine Rex brass checkvalve to minimize the backward seeping.
Basically, when the syringe is filling up, all liquid should be pulled out of the container, and when it empties out, all of it should go into the tank/supplying line, nothing back into the container.
So the trick is to find 1) fairly good checkvalves and 2) have the level of solution at about the same height as the point of injection.
I have combined this one with the waterpump macro dosing. The fluidoser injects the micro solution via a "T" into the macro line, and then once a day all of it is flushed into the tank (see post #26). Works well, especially if you have to place this under your tank into the stand, pumping it up to the tank would probably not work due to the slow seeping of check valves.
The inventor of this suggested to try different checkvalves, like those tiny ones that are used for nasal sprays and such, but I haven't found an easy way to implement them.
Getting a patent for something like this, good idea, but pretty expensive. I would have to sell a lot of those to make it worth... For now, I'll keep my day job.