The Digimax Digital Thermometer for Hydroponic system or Aquarium.
Simply place the probe in your tank and the unit outside to view the easy to read LCD display..job done!
Do you know the temperature of your nutrient tank?..
The ideal nutrient temperature range for healthy plant growth is 18-22c Anything under or over this range can be detrimental to your plants growth ability, and so you may need a tank heater..but how would you know?!