If vaguely recall hearing that a heater is most effective placed horizontally, (because heat rises and this distributes it better), and placed near the filter, which can help circulate the heated water. I think it might have been Lotus who said that, but I don't recall where. I would say that if you put it horizontally, don't bury it. I'm pretty sure they need to be exposed and not touching anything. They can also be kind of fragile and burying it with rocks is probably not the way to ensure a long life for your heater.