I know this thread was started a couple of months back, but I'll share my experience:
I purchased the SL2v2 & new salinity probe. Within 2-3 months, my readings gradually got lower and lower. DA replaced my salinity probe. Same problem started happening with the new probe. When I went to recalibrate it, I set it at 35 using PinPoint calibration solution. As soon as I would stick the probe in the tank water, it would jump to a negative reading. I finally calibrated it in the tank water but the values kept gradually dropping. (Didn't matter if I set the standard at 35ppt or 53 ms/cm)
I was about to ditch the whole thing and go for broke with a Neptune Apex system, but DA asked me to send it both the SL2 & the salinity probe - they're going to take a look at it and repair/replace it. I'll keep you posted on the final outcome...