Using the Salifert K kit, I found that my K went down to 340 ppm. Estimating that my tank is 400 liter, I dosed 240 ppm of TM K solution. After about an hour, I tested 430 ppm of K. Thus, based on the ratio of 400/430, I found that the net water volume was 372 liter. This is based on the hope that the Salifert kit is accurate. FWIW, it was accurate in measuring the Fauna-Marine multistandard.