which sensor to use for salt water level

[Alexei.Polkhanov]

I have seen systems where they ran a dry (air-filled) tube down to the bottom of the tank and used the resulting air pressure (out at the dry end of the tube) to measure the water pressure (which would be proportional to water level at a known volume/area.) But that may not provide enough delta pressure to be useful for small volume vessels.

I would +1 that.

There is a version of air pressure sensor that works pretty well on large tanks < 10 m high. It is based on measuring a pressure of air in a tube or more specifically a vacuum in a tube inserted into a tank upside down. This method requires  read-out of outside barometric pressure as well. Differential pressure sensor (one side is out another in a tube) positioned on top of glass tube (Honeywell makes many of those) will work perfectly for any kind of water.

For fish tank I would probably use 4 (or 2) load cell solution by vandalizing cheap "stay fit" scale and placing sensors below the tank. It can be calibrated accordingly and nothing need to be in a tank itself.

[tautech]

Try this:
Attached .doc

The problem with that is it involves a DC path for the current to flow though, which will cause electrolytic action and corrosion.

Use AC, rather than DC, that way there is no net flow of ions and less corrosion.

Comments accepted, but with this elaboration: (added to doc)
Due to resistor values, current will be uA
Any corrosion will not affect circuit operation as the salts/oxides conduct.
Only 1 probe is constantly immersed.
Probes can be sourced as Stainless welding wire and therefore cheap if replacement needed.

Sensing circuit is very cheap to build, I suggest you rats nest it for a trial and evaluation.

[Zero999]

Yes it'll work until the wire corrodes completely.

The correct way to do it is to use AC, so there's no net flow of ions.