Also, when you say the salinity drops when you turn the "UV on" does that mean the lamp or do you stop the pump and the lamp when the UV is turned off? If you turn the pump off then and that's when the salinity probe starts working, it could be the pump leaking electricity. However, I'm more apt to believe what you mean is just turning the lamp off, but water still flows through the UV. In that case, I agree with others, in that the cords for the UV Lamp are they running close to your Salinity probe cord? Relocate the salinity probe to a different area as well. Also, I've heard to try inverting the probe in the water as air bubbles can get trapped on or in the probe. By having it in the water upside down air escapes from the probe much easier.
I've heard Salinity probes are not that accurate. I never ran one, and don't think I will for this reason. Heh. I actually got the glass hydrometer that you float in the water to determine specific salt gravity, and measure that against my refractometer. The glass hydrometers are supposed to be the most precise. Just a pain to use.