I've been using my Doser 2.1 to add 1000mL of kalk per day to my sump. I've only been doing it 2 months and it seems like I have already worn the pump out. I changed it with an unused one and it is working fine again. In total I have pumped around 7 gallons through it. Is that a reasonable life expectancy for one of the pump heads? Would this be a better application for the Maxi? If so, when will they be back in stock?