USAMZ909&trade;Water Testing Pen TDS Efficient Accurate Tool for Testing Drinking Water Fish Tank Water Swimming Pool Water Well Water Aquaculture Water With slim line pen-style design, it's easy to use and store. Highly convenient, efficient and accurate. Display: Easy-to-read screen. Hold Function: Saves measurements for convenient reading and recording Hold Function: Saves measurements for convenient reading and recording. Great for water filters, drink quality monitoring, pools and spas, aquariums, hydroponics, etc.