JBL ProAquaTest K Potassium RefillFor perfect plant growth: determine the optimal potassium level to control fertilization in freshwater aquariumsQuick test: fill a test tube with test water, add the reagents, pour the cloudy test water into the special test tube until the cross on the bottom is no longer visible, read off the amount filled. This figure corresponds to the K in mg/l. Too little potassium causes plant growth to stagnate, even in the presence of all other factors such as CO2, iron and light. To increase the potassium content, use JBL Ferropol or JBL ProScape K Macroelements. At JBL/Aquarium Topics, you'll find detailed info on water testing and solutions to problems.