Reed switches are magnetically operated components with contacts hermetically sealed in a glass capsule.  Bringing a permanent magnet to the switch or placing the switch in or near an electromagnet causes the contact "reeds" to flex and touch, completing a circuit. Protective inert gas or a vacuum within the capsule keeps the contacts clean and protected for the life of the device.  These very small, inexpensive devices provide millions of switching cycles in normal circuits. Any time a contact must be made, without mechanically touching the switch, the REED SWITCH is a superior choice.

In brief, no aquarium water ever contacts the switch itself.  A small ring magnet within a polyethylene float ascends or descends to cause a magnetic field to flex and operate the contacts of the switch. 

Solid state power modules or as they are sometimes called, solid state relays, are actually devices that provide power switching through a solid state device known as a triac or thyristor.  A small gate current many times less than the actual power the device is switching is used to turn the triac on or off..  This miniscule gate current going to the reed switch within the water, rids the user of any shock hazard.