I run the 3172 and this only happens when there is too much CO2 or too low of a drip, I have always been able to solve it by reducing the bubble rate or increasing the drip rate. Keep in mind it is very efficient on CO2, I generally find one bubble every 2.5 seconds and a drip rate where the drip is almost a steady stream is about right, for my 300 gallon system, it then holds at a KH of 11 and Ca of 460ppm.