Having trouble with Fluval M series 300 watt heater

[Quietstorm]

I've been having trouble with the Fluval M series 300 watt heater. I bought it less than a week ago for my red eared slider. I know the heater is meant for a 80 gal tank, and I currently have a 29 gal tank - but planning to upgrade to a bigger tank within the week and didn't want to purchase a small heater, just to end up having to purchase another one.

The Fluval heater keeps turning on and off within seconds and does not heat up the tank to its set temperature.

Is it possible that having such a high wattage heater in a small tank could be the cause of the issue ? Has anybody else had issues with this specific heater ?

 

[CasperOe]

I am unfamiliar with the particular heater but I generally do not recommend heaters with build in thermostats! Rather have a look at a heater with no thermostat (example the BRS titanium heater) with a seperate heater controller!

And dimension your heater to the size of tank you wish to heat! Saves money on the electrical bill as well!

 

[TheBear78]

Is there much flow past the heater? I would assume that any heater, especially one with a higher wattage would be capable of heating the surrounding water quite quickly if it's not moving much.
This is another good reason for using a separate controller with a remote temprature sensor.