Rusty spots are getting worse

How's the pH? Raising slightly might help, but looks to be more than just a slight issue. Go to 6.3-6.5 in coco coir. 5.9 pH could be the entire problem, so i'd start there first as it's a fairly quick adjustment. This will help with Ca and Mg uptake. Also, start calculating PPM you apply from guaranteed analysis labels -- even this time. It can help rule things out by simply typing values into a spreadsheet or online ppm calculator. i have a nutrient spreadsheet linked in my gd profile comment section, no guarantees it works for liquid nutes though.. .think i have it properly considering specific gravity? if P/K are greater than 60 / 220, respectively and likely too high at each point, you might consider lockout to be the cause, at which point adding more Ca will only exacerbate the problem by locking out even more nutrients when Ca goes too high (not necessarily where you are at, but possible future outcome without enough info...) had to re-write this after seeing the pH