Browning tips help

ive noticed these really light browning of tips on my critical jack auto seedling what does this issue look to be related to? ive just recently put a layer of MG perlite and play sand because of a gnat issue the mG did have nutes in it but realllllllly low NPK. wanna make sure this isnt going to be an issue that spreads , i wanna correct it before it becomes worse
i also waterd with 3 percent hydrogen peroxide in the ratio 1.25 table spoons to a gal

pretty sure the 'play sand' is your problem.

Ever since I added riverbed sand to my grow everythings gone downhill.

i removed the sand , i tested my ph its about 6.5 ish i dont understand