I think few brands suggest not to use heater because of the calcium concentration. Higher calcium contrasted salts are exothermic and makes the water hot anyway and adding extra heater can cause cloudy water. Using heater after mixing shouldn't an issue and hence once the mixed water goes into the tank with heater, its no longer their concern :P
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You wouldn't want to see my tank. I don't use fancy equipment and I am a noob 
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