Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Oh, OK, then I see what you are saying about fluid in the tube draining back.
As the supply or stock gets lower the pump may pump less each time, but if you have set it up and ran it from beginning to end of a cycle and actually measured how much really ends up in the tank, that is how you figure out the dose. The fertilizer remaining in the tube is not part of your measurement. Yes, it will vary as the stock volume varies.
You could set it up away from the tank, as long as the height from stock to tank is the same, or you could run it on the tank, but catch the amount that comes out.
If the stock bottle is above the tank then the siphon could run the other way, so I would keep it below the water level in the tank.