This is how I made a simple, cheap dosing pump for myself.  I use it to automatically add the proper amount of plant fertilizer every day.  Currently I only have to fill mine up once every three months.  Laziness is the mother of invention!

Components

Kitty litter bucket: holds 4.5gal and is square which makes it easier to use suction cups to support the spray bar.

Pump: Small submersible hobby pump with 1/2" outlet adapter (I used a Aquarium Systems Micro-Jet MC 450).

Tubing: I use 1/2" from the pump to the spray bar, with a tee in between that connects to an adapter that goes from 1/2" to airline tubing.

Valve: Airline tubing valve to control flow rate.

Spray bar: This makes sure that what ever you want to add to the tank is in solution and has not settled out.

Controller: I use an X10 appliance module; but any programmable digital electronic on/off controller like they sell at Radio Shack for turning on/off table lamps would work since they have one minute resolution.

Example of Operation

1. Fill the bucket with just enough water to cover the top of the pump.

2. Run the pump until it stops pumping water, turn off the pump and measure how much water is left in the bucket (m).

3. Fill the bucket with four gallons of water and mark the top level (v).

4. Set the timer to turn on for one minute and measure the output created by one minute's pumping (o), assuming the valve is wide open -- if this volume is too much you can close the valve some, or it the volume is too little you can restrict the output of the spray bar by closing some holes or putting a valve between the tee and the spray bar.

5. Convert m, v, and o to like units and calculate the number of minutes pumping it would take to empty the bucket: (v-m)/o=p.

6. Determine the volume of additive you want delivered to the tank every day (a).

7. Calculate the total amount of additive to put in the bucket a*p=t.

8. Fill the bucket with water up to the mark, and then take out an amount of water equal to t.

9. Add your additive in amount equal to t.

10. Set your timer to run for one minute every day and mark on your calender to check the dosing pump in p-3 days.

11. Enjoy your aquarium!  :)