Happy frog run off PH very low, never fed plants

Title says it all.. planted seeds into happy frog.. plants look OK 2 are fine 3 are ok showing different issues. But I have NEVER fed anything to these plants... just water... I always phd it at 6.5 and before I even planted the seeds I watered the soil heavy to try and lose some nutes.. what the hell is going on??? Plants are now around a month old.. and today I had to water at 7.6 to get a run off of 6.1???? PPM of runoff is between 400-1000 higher then water going in... someone please help me.. since I started growing again I have had issues with almost ALL plants in veg.. my 9 in flower look great super frosty.. but veg constantly having problems?

Temp 75 Rh 40-60 water 6.5 24/0 light

My water is tap water that I have in a 55 gallon drum with air stones.. so the water is out for weeks

Could be a bad batch with very little buffer lime in it.

I have had issues with almost every plant grown in it during veg only, flower I have picture perfect plants for the most part a few are from same bag of happy frog

What do the plants look like? Runoff pH is useless. Runoff EC is not and yours would undoubtedly be low. pH it going in and watch the plants.