Well here's another update on the system. I am still using it for micro dosing (haven't had time to mess with setting up another bottle for macros yet). I settled on what I call "the final design" because it has been in operation the longest amount of time without me fiddling with it. Here is a crude drawing of it as it is now.
Since the last time I checked in I've made a few mods:
1. Revised dosing control: I have decided to use a $15 digital timer to turn the pump on and off to control the amount dosed to the tank. Instead of restricting constant flow of fluid with valves I'm now letting the flow go unrestricted and just controlling the amount of time it goes into the tank (like you would do using a traditional dosing pump). This takes care of noise issues (don’t have to listen to the pump all day long), wear and tear issues of the air pump being on 24/7, and eliminates high pressure build up that was present on the old design. I'm currently setting the air pump to run for 2 minutes.
2. Larger reserve: The system now uses a 4L wine jug. This is the type with the screw on top. (Actually they are identical to the sobe bottle tops.) The main reason for this was because the unrestricted output (remember I removed the needle valve) more liquid could pass in a given amount of time. The shortest time interval the digital timer could do was 1 minute intervals. Probably 40 sec interval would be perfect, but I had to improvise so diluting the fertilizer in more water was the solution. Not a bad thing (think of it as a auto top off too)
3. Solution mixing: Sometimes when the trace solution sits things settle to the bottom so I decided to add an air-stone to the solution bottle. Before air just ran into the bottle above the solution level just under the cap, but now I've run a airline to the bottom with the air-stone so when the air pump goes on it agitates and mixes the solution up a bit. A concern with this is oxidation, but it is only for 2 minutes a day- I doubt that it is enough to really do anything, but mix the mixture up a bit. One thing to note is that a check valve should be used on this airline or else the mixture could siphon through the air-stone and to the pump.
This system has been in operation for the last 4 months without a single problem. Works extremely well at this point. Pretty much just refill the bottle after my water changes on Saturday and it's empty by Friday afternoon.