Soil pH way to high

So I started 2 plants in premix seedling soil and they were doing fine... Transplanted them into promix bx and 2 weeks later they look horrible ... Been using nutrients and water pH to 5.8 ... Decided to get a cheap soil meter and it's sitting at around 7.5-8 ... Went and checked the bag of soil and its also sits at 7.5 - 8 ... What am I doing wrong??? ... Do I have shit mix or am I not feeding them properly ... I've noticed with the nutrients I have I'm using a lot of pH down to get it to sit at the right levels lol also using filtered water atm.

RO filtered? you may have to cut it to 1/5th tap to allow the PH down to have something to bind too. water that is totally clean is hard to keep stable, any other source or alkalinity?

also make a slurry test for ph ditch the cheap meter

Promix is ph buffered to around 6.3

No need to ph your nutes at all