Heaters in reef aquariums

The amount of watts your heater will need to heat an average aquarium with a small sump is roughly 50 watts per foot of tank, sort of!
With a large sump with a substantial water volume, slightly more!
If the heater is the glass type, never submerge the head of that heater and never put it where the water level can drop, the glass needs to be in water always!
If it gets hot out of the water, let it cool before putting it in water or your tank may be chlorinated!
When hot glass hits cool water the glass usually cracks and lets in water.
When a charge of electricity is free in your water, like when the glass in a heater is broken letting water in, it will break the bind of the components of the salt, being sodium and chloride, chloride is chlorine!