well my containers would only be 500ml, and i would be thinking low doses like 20ml doses so that would last quite a while, and there wouldn't be much head difference. It just has to be close, it's ei anyway, and i don't mind adding a little more hear and there when i have to, it's mainly for when i go away the tank will be getting everything it needs, even if the dosage is a little shy, it should be fine more than fine if i go away for a week or two. Plus i can just test it every week and see where i am at and adjust. I don't mind that at all. I need 4 pumps or i would get the peristaltic pumps, but that would get expensive.