Yesterday I came up with a DIY liquid doser for fertilizing the tank and constructed a prototype. Here's how it works:
A section of tubing is submerged in a jar of the liquid to be dosed and connected to an airpump. A small hole (I used a hot needle to poke a hole in a 'T' connector) allows the fluid to enter the tube and air to bubble out when under pressure. The airpump is then connected to a timer so that it runs for about 10 minutes/day.
What happens is that when the pump is off the fluid level inside the tubing equalizes with that inside the jar. When the pump turns on, the fluid is pushed up the tubing and into the tank by the pressurized air (it burbles like an espresso machine). The pressure in the tubing also bubbles out the inlet hole and prevents more fluid from entering the tube. Once the fluid has been pumped into the tank, air flows through the tubing until the pump turns off.
The amount of fluid pumped is equal to the internal volume of the submerged tubing. Standard airline tubing is 2"/ml, and mini airline tubing is 8"/ml. I found mini tubing works best for the coil and tube running to the tank.
A tiny hole is needed in the lid to let air in as the fluid is pumped out. We want both holes to be small so that the pressure in the jar is enough to push the liquid up to the tank. A check valve in the lid may work extremely well for this. If the doser can be mounted behind the tank at tank-water level, it's very easy to get enough pressure to pump the fluid. To pump from under the tank required some trial and error, but two small needle holes worked well for me.
As the fluid level drops, the volume of submerged tubing decreases and the size of your dose gradually drops. I compensate for this by coiling the tubing at the bottom of the jar and keeping the inlet and outlet tubes vertical to minimize the volume loss. This reduces the effect and a short, wide, pickle jar would work best.
I ran a bunch of tests into a test tube, with about 8" of mini tubing and 3" of regular tubing, I got 2.5ml with about 0.1ml accuracy.