Potassium deficiency?

Hi all, I was like 80% sure this is a ph imbalance that's causing potassium deficiency but i would really appreciate a confirmation and also to check that i am taking the right steps in fixing it.
I'm in a 2'x4'x60" tent using a 300w (really 146w) marshydro light that i keep about 20 inches away, temps range from 68-86 with humidity being around 45-60%.
Soil is black gold seedling mix, before this happened water ph was 6.5-7 with runoff being 4-5ph, using 350 ppms of dyna gro nutes(ruled out nute burn cus of low ppm from my research..).
Once leaves started showing pale spots near the edge i flushed with 6ph water until runoff was 5.5ish (using liquid ph tester), no nutes. Then waited two days for soil to dry up a bit, leaves got worse at the tips so i added 400 ppm nutes. I thought everything seemed fine but now i see a little discoloration on a lower leaf..

Thanks!

Nute burn and why are you feeding already?

I thought when the cotyledons die off you should start feeding.. also the soil is just peat moss, perlite and dolomite lime so it seemed right to me but i guess just bad info. What would you suggest? flush with no nutes and leave it? Was thinking about re-potting it, should i hold off?