Kleen, I think you are barking up the wrong tree, so to speak, in trying to determine dirty aquarium water by resistance measurement.
All the aquarium test kits I ever came across measured the PH of the water, fish are far more sensitive to that. This assumes you are talking about a fresh water aquarium, for salt water I have no personal experience at all.
Now, you should have an aerator pump and also a carbon filter to filter & clean the water. Maybe you could design something that detects when the filter starts clogging up and needs changing. The water can stay in there for very long times if your filter(s) work well. You should not change all the water at a time anyway, it *really* upsets the fish.
What I did was siphon a portion (maybe 1/4) off at regular intervals, say once a month, and replace it with water that had settled in a container for a few days.
Just keep in mind that fish are sensitive to stray electrical currents when you poke powered devices into the aquarium :shock:
Klaus