Oh man, be careful putting your heater under water if you're not sure. It could ELECTRICUTE all your fish as well as your self. Some heaters are not submersible. Mine is glass and is not submersible. Hope you read this.
Heaters are one of those things you are better off not taking any chances with.
If it's a hang on the back type heater it will have a device on the top to clamp it on to the rim. There will probably be a screw to tighten it down with.
A submersible will look more like a sealed tube with a wire sticking out of it.
If you are in doubt, take it back to the lfs and compare it to the ones for sale.
It's the top picture. Sticks to the glass.
eco therm heater 100W.
Had a look on the net but could not get any help.
Hve not put it right under water yet.