Me and My Girls need Help!

I am in my 4th week on a grow of NL's and I have been chasing a problem that started out as a few rust spots on a couple leaves on one of four plants in a Eco Grower Hydro/DWC/Drip system. The spots are now on a couple of other leaves but only a few. I could give you a history of all the steps I've taken but at this point I figured to give the experts a few basics and pics to generate comments and questions. PH has averaged 5.3 with little swings; HG Flora Series @ PPM 720 (.50 Hanna); Solution Temp 72F; Tent Temp 79F & 55% RH during Day & 69F & 40% RH during night time. I can provide additional info as requested. Thanks

Do you use cal /mag suppliment?