I don't think there is such thing called ideal conductivity for fish. Conductivity is a measure of dissolved ions. It's more important to know what ions are in the water. Soft water has low conductivity due to absent of many ions, and hard water or salt water has high conductivity due to present of high concentration of Ca or Na ions. Conductivity will go up in any water if you don't do WC as nitrate and other dissolved ions from food waste will go up and is bad for fish. I have a conductivity meter that I used to monitor my tank water. The only use is to tell me when WC is over due when the readings are sky high as there is rough correlation between conductivity and nitrate level with the same set up.