How to replace heater in tank?

[KyleKVK]

I recently got a new heater to replace my old one since the new one has an adjustable temperature. 
 

But how can I replace it? Specifically, how can I get the suction cups to stick to the back glass with water in the tank?! I know it sounds silly, but I’m serious. I can never get them to stick well with water. 

[FrozenFins]

I've had the same issues. What I did was the drain the tank a bit, and suction the heater to the glass where there was no water, and then slid it down to where I wanted it.

[L.W. Wetarm]

I think mine is hanging by the cord most of the time. I used to try to mount it horizontally but gave up on that pretty quickly. 

[JettsPapa]

I don't have any problems getting them to stick underwater, but there are a couple things to watch out for.

• The glass has to be clean.  If there's a thin layer of algae or anything similar they won't stick.  Take a razor blade scraper and clean the glass first.
• I learned the hard way that they won't stick if I press too hard.  Just moderate pressure and they stick fine.