Potassium deficiency need help

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Hey guys so this is my first grow ever and it appears my plants are experiencing a Potassium deficiency but I believe it's probably nutrient lockout due to improper range of PH. I watered my plants last night after trimming off some of the leafs that had been burnt to a crisp and it looks like in the past 24 hours more leaves are being slowly affected. Got some pictures here for you guys but just curious what i should do I just got a PH meter today but I dont think it would be wise to water again to find out the runoff PH since I need to let it dry out a bit first. The soil is FFOF with about 1/3 perlite added

 

[Vanquish]

I haven't added any nutrients and it's only been watered since seed. Im just curious what you guys would do at this point I know the pictured aren't the best i could take some more without the reds / blues if that would help. I do have some potash I could sprinkle around it but im not sure if that might do more harm than good not knowing if the PH is right im using cub foods purfied drinking water right now because I have a water softner and read that the salt gets into the plant an causes issues.