Directly from the aquavitro website:
When I mix salinity salt in my water barrel using a pump and a heater I get cloudy water. What is happening?
When mixing this salt, we have found that 24 hours or less is ideal (more specifically 6-10 hours is best). The longer you let the salt mix, the more CO2 it will take on and the more precipitation you will encounter. CO2 --> water --> carbonic acid --> bicarbonates/carbonates. Mixing the salt for more than 24 hours can cause a decrease in alkalinity and pH, resulting in a precipitation of calcium and carbonates. When we mix it here at Seachem, we mix it in 50 gallon drums with one power head at room temperature (22-25 degrees Centigrade). There is no need to use a heater or multiple power heads. As a matter of fact, we do not recommend using a heater or powerful circulation when mixing Salinity. Typically the water is clear within a few hours but sometimes we have to use it within an hour of mixing, when it is still cloudy. This cloudiness clears rapidly once introduced to the tank, generally within 30 minutes, and will not cause any harm to the inhabitants. Furthermore, cloudiness seems to persist more with buckets that have calcium and alkalinity levels on the higher end of the range. This is really inevitable when you have such high levels of those particular components. I would recommend that you check the strength of the pump and reduce it if necessary, and to hold off on using the heater until the mixing process is complete as the heated water will create higher likelihood of precipitation.