The ~$180 sensor was an ion balance sensor probably. This uses a platinum
electrode, a very thin ion-transfer membrane and another electrolyte, and
another platinum electrode. This is not a sensor for unattended operation,
it is a lab device (also used to test ponds, aquariums etc). Both the
memebrane and the internal electrolyte get contaminated after a while and
the readings will drift away... fish soup anyone ?

I don't know if a more robust version of this exists.

Peter