Simple old school Mercury thermometer suction cupped to inside right corner of tank. The first thing I look at every time I walk into the sitting room. Simple but essential piece of kit as I dont use probes. Use hanna sg/temp tester with weekly water change but with changeable Irish weather a quick readout for hands on mixed reef keepers like myself is in my opinion invaluable. I know eyebrows get raised about having a potential Mercury leak into the DT but proper care when cleaning it and changing it for a new one every couple years for me is enough.