I have a adittional in-line device on my gas boiler on the water radiator side that filters rust using magnet tek
Might be something to consider ( diy ) I think car alternators from car scrapyard have powerful magnets in them 
In my feed I like 2.5-3ppm iron , so a little is good ( calmag with added iron ) my water is soft with no or very little iron so I must supliment it
Yellowing at new growth tips is iron def
You would need to look at circle antagonist diagram to see what too much iron aggravates so to know what def to look out for ( @George may be able to help you with that )
Do you drink your water ?
: )