A water monitoring device monitors and maintains swimming pool chemistry. The device mixes reagents with water in flowcells. The water chemistry is detected by measuring the light transmitted through the flowcells. The water monitoring device can communicate with computers, servers and mobile computing devices which can store and display the water chemistry information. The reagents can be stored in a replaceable reagent cartridge which can provide reagents for water testing and can be replaced when the reagents need to be replenished.