TBH, I always tell people to get a thermostat because it makes me feel safer. But I kept a Leopard gecko for 8 years with nothing but a stick on heating pad and a humid hide in a 10 gallon and he was fine. I wish I woulda never gave him away lol.
In an aquarium the heat dissipates much quicker so as long as you check on your geckos daily to make sure that they aren't all hovering on the cool side (indicating it might be too hot and you should check on it.) then the chances for a crazy burn aren't quite as likely.