The Flowmaster by Hydrologic measures how many gallons of filtered water have gone through the your grow system and alerts you when it's time to change filters. You can program in the filter capacity and when you reach the target number of gallons, the Flowmaster beeps. This is important for knowing when to change the pre-filter for the Merlin Garden Pro. It also ensures your system runs at its peak efficiency for removing contaminants. Available in 3/8" quick connect for the Merlin Garden Pro & Tall Boy.