The U.V tube in our units basically a specially designed T5 tube and as such it will generate some heat energy, the same with all UV units, which will go towards the total heat load on the tank , other devices such as pumps and lighting will also add some heat.
This cannot be avoided but can be offset with the use of a cooling device such as a fan and making sure that your temperature controller is set correctly.
If excess heat is coming from the unit it is also best to check the water flow rate going through the UV. If this is slower than rated the water temperature can be affected more. When rating a pump to supply water always take into consideration the head height that the pump has to work against as this can make a dramatic difference to the actual water flow.