First, I am not really sure you need a heater. Where are you keeping your turtles and what is the water temp? Most of us do not use a tank heater because the turts only need their water to be between 70 - 80 degrees. As this is the normal tempurature of an air-conditioner, this will be the tempurature of the water. Now some people do use heaters and set them to about 75 degrees, and there is a chance that if a turtle touches it they will get burned. This depends on several factors, but it all boils down to one thing in my opinion: can you hold onto it with out getting burned? If not you will need to create a protective barrier.
Off the top of my head, I migh use plexiglass glued into the corner with aquarium glue. Drill some holes or leave a gap at the top and the bottom. Then by convection it will heat the whole tank. Cold water enters through the bottom, as it heats it rises and comes out the top, and the process starts all over again.
If it seems like I am overly simplifing this it is because I have no way of knowing how much info you need so I just try to cover all the bases. Hope this helps. If not ask for more.
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Joshua M Guillemette
1.1.0 Children [Deven 10 & Hailey due June 1]
2.1.0 Cats [Micah, Judo, & Elizabeth]
1.0.0 Bunny [Bugs]
1.0.0 Red Belly Slider [Bernulli]
2.0.0 Yellow Belly Sliders [Maxwell & LaPlace]
0.0.2 Cumberlands [Pascal & Joule]
1.2.1 Red Ear Sliders [Kepler, Napier, Gauss, & Kelvin]