The Apex probe measures conductivity and mine has been working fine for 2 years.
There is a place in Fusion to adjust temperature compensation.
Then compare to your refractometer to dial it in.
I check the tank with my refractometer about every 3 -4 months to compare and it has always been dead on.
It can be useful to pick up extreme swings and send you an alarm.
I've been using it for a while. If you have the correct solution, it is easy to calibrate. As long as you have a refractometer, you can always verify it's reading. Mine lasts a while before it needs to be recalibrated.
I never saw any reason to have the salinity probe as unless something catastrophic happens, salinity shouldn't really change day to day if you have an ATO.
i have been using it on my system and it has worked very well, i have checked it with both refractometer and a digital refractometer and it stays real close if not right on. it did take me a bit to dial it in and you have to make sure there are no bubbles in it and my probe is in the sump with the skimmer so if the skimmer goes nuts it will get bubbles in the probe and mess it up.
other than that it works very well, the only probe i am not sure of and need to evaluate is the orp probe...
I have one and thought it would be really handy in detecting a drop in salinity in case there is a slow tank leak or ato malfunction; however, I find it quite useless for an accurate reading. I find turning my pumps and return off and floating an old school Tropic Marin Hydrometer is the way to go to get a real time accurate measurement of salinity.
I guess it still would work to detect an ato malfunction or lead... maybe it is worth it just for that? Only you can answer that.
i have the conductivity probe and find it useless. I have an ATO and really no way for my salinity to change. Same readings everyday. If i did some selling coral it could be nice to see when it needs bumped back up