If it were me id drop your ppm to 600 and keep your pH between 5.8 - 6.2
It might sound counter productive but they drink better and the nutrient uptake is better , and id add a dash of bleech to your resevoir to prevent future issues if they havent already started . And what sort of hydro system are you running ?
A flood and drain
Deep Water Culture
Recirculated Deep Water Culture
Etc
