Thanks againI have mine situated horizontally- even in my other tank that has deeper water. It works fine. I watched a video from Aquarium Co-op on YouTube that recommends mounting your heater either horizontally or at an angle ROUTINELY.
Their reasoning is this. The more a heater cycles on and off, the sooner it will wear out or malfunction. When it's mounted vertically, the warm water immediately around the heater rises and the temperature sensor, which is in the head or the top of the heater, senses the heat and so cycles off. The water around it cools a bit, so it cycles back on again, etc etc etc. But if the heater is mounted horizontally or at an angle, MORE of the water is heated before there is sufficient heat in the tank for the sensor to cycle off. The idea is that a vertically mounted heater cycles on and off a lot more often than a horizontally or angle mounted heater, so it will wear out sooner; the horizontally or angle-mounted heaters will last longer.
I'm no expert, but that logic makes sense to me. So all mine are mounted horizontally.
Yes, Millie checked out her heaters both before and after I put the guards on them. The gaps in the egg crate are much too small for her to get her head or foot into. If your musk turtle is an adult, then there should be no problem; Millie is 4 inches long. Gary sounds like a curious little guy- kinda like my Millie is! that's part of what makes them fun.
I hope this helps! And don't apologize for asking questions!!!! Wanting to learn is a GOOD thing!!!!!!!