I just bought a water heater....one of those submersible ones. The water line indicator seems to be close to the top of where I would clamp it on to my tank. I guess if i were to fill the water to the water line indicator, it would be about 2 inches from the top of the aquarium. What I'm wondering is, if the water level is so close to the top of the tank, how is it possible to put one of those Turtle Docks (with the suction cups) just above the water level without the turtles basically......escaping? Since everything is so high? I don't have a cover right now. But then I think if get one of those flat "screen" tops, the turtles would be hitting their heads on the cover???

hahah I'm kind of confused. Maybe I'm picturing it wrong.
Is it a must to have the water all the way up to the water line indicator?