Heaters used in a reef aquarium.

If they are of the design where the wires come in at the top, the controller/temperature dial is at the top and all set in rubber or silicon, then in the factory that rubber section is forced in, then time will show you that this does not last and it will leak!

Fist up it is at an amount that you can not see, but you will see the loss of functions!
That tube shape, where the heating is achieved then radiates out thru the glass, if it is the glass type, in there it is heated, then not via thermostat control. Well that action creates expansion and contraction of the air in side there and in time your heater is no good any more and first up it looses its capacity to regulate thermostatically the heating function thus cooking or over cooling your tank life, then its all down hill from there!
Yes, they are sold to sit submerged in the water, many things in life are made to do a job and used in a certain way, but in so many occasions, if you vary how you use those many things, they last a lot longer!!

I understand that many reefers have not done much, or had to endure much with this hobby so if you do it the exact factory recommended ways, this hobby could use your money directed to the shops via dead tank life and a new heater, in this case.