Planted Tank Guru
Join Date: Jun 2004
Location: Baton Rouge, LA
How about a reciprocating/plunger style pump?
There's one shown on the page Wasserpest linked.
Use a syringe as the pump. Connect the nozzle to the center of a standard airline tee. Connect the two ends of the tee to the reservoir and aquarium via two check valves, so that solution always flows the proper way when the syringe's plunger is actuated.
You just need to pull, then push, the plunger.
If a mechanical timer doesn't suit or isn't strong enough, use a good servo. That would allow you to program in any dosage.
Or use any gearmotor with sufficient force, like a <$10 cordless screwdriver. Just like the horn on a servo, attach a disc to it, with a rigid mechanical linkage from the plunger to an off-center hole in the disc. There's no need to reverse the motor, which simplifies drive; all you need to do is turn the disc through one full rotation. A full rotation can be sensed with a microswitch that detects a protrusion or cutout on the disc. Dosage can be adjusted by the throw of the linkage, number of full rotations, and concentration of the solution. Just one digital input and one digital output (with a drive transistor) on an MCU for this. Easy.
With a little shopping around for inexpensive gearmotors, I'd think this could be built for $15/pump.