Oozleanswered grow question 2 years ago No 1 will be able to give you a proper answer without more information.
For a proper diagnosis you should have: Pictures of the whole plant, roots, detailed feed information, how much base nutrients, type/brand of nutrients, how much cal-mag, last res change, pH range (cause I don't believe it has been perfect 5.8 the whole time). What's your environment? VPD can cause calcium issues. How close is your light?
All that info will help majorly in diagnosing and fixing your problem.
This site is designed to store all this information. You get back what you put in.
And EC 1.2 doesn't equal 900 PPM with any PPM/EC conversion scale (US/Eutech/Truncheon), so typo maybe?
Good luck