low salinity and diatoms

[Paolissimo]

Hi guys, i have an issue that has raised in the last few weeks. From the group buy I bought a Hanna phosphate test and a calibration fluid for my refratometer. The reason I did this was because my tank in the past few weeks has not been doing well. I think i have dynoflagellate. I checked phospate with the hanna and is high, 0.12. After i calibrated my refractometer (i originally calibrated with both ro/di and distilled water)i realized that my actual salinity is/was close to 1.020 Yes that low. Also when the tank started changing, I switched salt from reef crystal to instant ocean. Can the lower salinity, high phospate (can't check nitrate because I want to get a salifert test)and instant ocean salt have anything to do with dyno? Or was it probably introduced with a frag?
Thanks
ps i know the subject says diatoms, i meant Dyno