The Dennerle Dosator is a clever, reliable and inexpensive way to dose plant fluid fertilizers such as Dennerle S7, Dennerle V30, Plant Exixier, very evenly and constantly into aquarium water. The associated diaphragm "Membrane" or metering ring wears out over time and should therefore be regularly renewed. 

Membrane should be replace when white membrane turn color or every 4 weeks.