Hydor Theo water heaters allow you to keep your nutrient solution at the optimum temperature for your plants. Fitted with a thermostat, they can be left on continuously without any need for adjustment.
The heating element is made of a special polymer that has been silk printed with special ink, the ink heats up with the passing of electric current, and the heat is transferred to the water.
Other features include special shatterproof glass, a precise temperature setting knob that clicks in 1°F increments, an on/off indicator lamp, and it switches itself off if the heater is not fully submersed to prevent overheating and damage.
They are available in 50w, 100w, 150w, and 300w, depending on the size of your reservoir.