Tetra 150 watt heater flickering

[sinibotia]

Just set up the 29 gallon tank (from a tetra kit) with my fiance but the heater seems to be cutting in and out. Is that normal, or is there something I can do to fix it? Im going to be really irritated if its defective and I have to wait for them to send a new one....

Edited to add: the tank was filled with cold water, so its not just flickering to maintain the temperature.


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