I am currently testing a similar design. For my pump I bought a Tom Aqua Lifter AW-20 vacuum pump. It looks like a small air pump with two connectors that are labeled in and out. It is not submersible. It is rated at 3.5gallons/hour and lifts fluid up to 30". It only cost $9.99 at Big Als Online. For my reservoir, I'm using an old plastic gallon jug. I drilled a hole in the cap and stuck a piece of rigid air line tubing through it to reach the bottom. I put a couple additional tiny holes in the cap to allow air to enter as the fluid leaves the bottle.
I set the bottle on the bottom of my stand next to my 20gal High sump tank. I set the pump on top of the sump tank. I used flexible airline tubing to connect the rigid tubing to the pumps input. I connected another piece of flexible airline tubing between the pumps output and a hole drilled in my sumps lid.
I used a stopwatch to measure the amount of time it took to fill a 250mL measureing cup. With the reservoir full it took about 4.75 minutes, and with the reservoir near empty it took about 3.25 minutes. I averaged these times and calculated that it was dosing at an average rate of about 64mL per minute. I figure if I set a timer to turn it on for 3minutes per day it will dose almost 200mL per day, and that 3 Liters of fluid in my reservoir should last just over 2 weeks. The variation results in an error of about 20% in dosing, meaning that it will dose about 20% more than the average the first few days and about 20% less than average the last few days. I figure that is consistant enough for my needs.
Now only time will tell how well this system will perform in the long run.
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