Interesting idea, can you give more specifics about the pump you used and volume of the container etc.
I recently tried something similar that didn't work very well. I used a 1 gallon jug that had two holes drilled in the cap which were fitted with airline bulkheads. To one of the connectors I attached a rigid airline tube to the inside of the cap and a hose that ran to the tank on the other side of the cap. To the second bulkhead I attached a hose that ran to an air pump. I figured when the air pump was on it would basically blow the liquid from the bottle into the tank. The problem was that it was very inconsistant with the dosing. It required much more time to send the same amount of fluid as the water level in my bottle dropped. This was probably due to the change in back pressure caused by the increasing height the liquid needed to be pumped.
I have been considering trying a gravity feed, and using a soleniod valve connected to a timer to turn it on and off. Just havent looked into the solenoid valves yet. I figure if this works it should provide a very consistant rate of flow.