Heater for 15g cube tank

I have a 15g cube tank currently cycling and I put a 100 watt Aqueon preset heater in it that is preset to 78*.

It’s been running for a four days now and the water furthest from the heater consistently measures at 68*. I’m frustrated because 100w is what’s advised for a tank this big but it’s not getting any warmer.

This is my first tank, so any advice or recommendations would be much appreciated.