Alright, let me give a bit more detail.
...
The purpose of this project is to be able to monitor the level of my top off tank for my salt water aquarium while I'm traveling.
...
Isn't the "larger purpose" to
maintain the level of the tank, not just monitor it?
What if you just had a sealed, raised, container whose outlet hose opening was put at the water level of the tank you are maintaining? Sort of like an automatic pet watering setup.
Sorry if that's naive, I don't know anything about salt water aquariums

Or, what about a toilet valve in an aquarium? Depends on your aesthetic sensibilities I suppose, but what a conversation piece!!
John