Rust Spots

[Platypusmann]

I'm an ebb&flow hydro grower with a degree of experience and successes.......I decided to start 5 feminized seeds (Nirvana Supergirl) in a 2 part sphagnum/1 part perlite mixture soil grow.
They are 2 weeks old an 8" tall, looking AWESOME overall. Fed them the first nutes (AN Micro, Grow, Bloom, Mother Earth Tea & Sensizym mixed at 1/4 strength) a week ago, and within 3 days of feeding they grew substantially, and are lush green.
However, yesterday I noticed two plants with rust colored spots on the first set of fans. It looks like nute lockout to me.
I have always used pH 6.2 water and nutrient mix. I have only been having to water 1x week. I water based on dry weight and wet weight. Today they weighed close to dry.
I flushed the pots with 2x pot capacity plain ph 6.2 tap water (ppm 103) today and runoff ph was 5.1, which is awfully low.
I thought sphagnum and perlite were pH neutral, so what do you think might be keeping the pH down like that?
Is there anything anyone would suggest I do to get the pH stable again?
Any ideas, opinions or suggestions would be GREATLY appreciated!!!!
Peace!!
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