You could also go with Wasserpests water pump dosing
which doses consistently, and you can play with the concentration of the solution to dose the right ppm's. All you need is a little water pump, they can be had for $10-15, or maybe you have a spare one sitting in your stand anyway.
Let us know if you figure out a way to get this cheap liquidoser to work... it has a lot of potential. One of these days I will start poking holes into airtubing again.
