Bill wrote:

Try a local radio dealer that sells "walkie talkie" type radios, most of
the whip aerials are stainless

Careful! Often nickel-plated brass is used. I would not want them in my
tank.

If I had to build such a sensor, I'd probably use pencil mines (carbon
and clay in various proportions), as they are quite inert. You can buy
them from an office supply store in various diameters from 0.1 to 1 mm.

One other thing to keep in mind. If such electrodes are supplied with a
DC voltage, hydrolysis of water covers them with with gas bubbles, which
act as insulator. This process is called polarisation. To avoid the
effect, a low AC voltage should be used.