The present invention relates to devices, methods, and kits for quickly and easily measuring and monitoring the pH of aquatic environments, such as lakes, streams, drinking waters, fisheries, aquariums, pools, hot-tubs, and spas. The current invention includes devices that are reusable and capable of monitoring the pH in a reversible fashion, thereby allowing continuous pH sensing without a need for the user to physically perform pH tests. Operation of the invention involves placing the device into the aquatic environment and optically detecting a color change in the membrane, for example, by looking at the device, when a pH measurement is desired. In addition, the device is both inexpensive and reusable, allowing the user to monitor the pH of various aquatic environment at minimal costs.