Heater problem/advise

[Herefishie]

I have a 15 gallon Fluval Flex. When I ordered the tank from Amazon I also ordered a Fluval M30, I believe it's called, 50W submersible heater. It is supposedly adjustable but it doesn't matter what temperature I set it at, it is constantly readings 80° when I test the water. Since the heater sits behind the panel in this style of Aquarium, I don't need one that has a hang off the back display, etc. I just want one that is adjustable and actually works. It's too late to return it so I'm planning to buy a new one. Every review on every kind of adjustable heater I read on Amazon with one star seems to complain about the same issue. Does anyone have any recommendations for an adjustable heater that actually works? TIA.

[Lennie]

Have you kept it submerged in the tank around 15 minutes to reach the temperature of the tank and plug it in afterwards? 

I usually use tetra heaters. For my newest tank, I got Aquael's 50w ultra heater. I have been using it for a week only, but so far I love its look and size. If I face no problems, I feel like it is gonna be my go to. It shows the temperature of the tank and how far it is from the temperature you set. Also visually pleasing imo.

This is the product:

This is the video you can see the product on:

 

[Ninjoma]

The 50W Aquarium Co-Op Heater?

[Mmiller2001]

Ink Bird controller solves all the problems one has with heaters. I will never not use them.