tips of top leaves yellowing

I am growing 5 og kush, 6 chemdogs, and 6 ak47's, they are about 6 weeks old on day ten of flowering, they spent the first 3 weeks in half gallon grow bags with seedling potting mix, then transplanted to 2 gallon grow bags of fox farms happy frog. The soil had plenty of nutrients for the first 2 weeks, after that, a few days ago at day 6 of flowering, they were dosed with a fourth tablespoon of maxi bloom per gallon of water. My soil had a ph of 5 so i have been watering with water at 7ph, i did just realize earlier that my last watering i didnt raise the ph of the water after adding ferts, so they were watered last at ph5.5...

For the last few weeks i have noticed the tips of the top leaves are a yellow color, only the very last eighth of an inch or so though, growth is still good, color is healthy, and my hps is about 12 to 18 inches from the plants, i dont think its the light because it effects almost all the plants, whether they are short or tall...i am assuming its a pretty basic issue but ive never had it before, this is the first time using happy frog, so i was thinking maybe it was just the low ph, i didnt add lime beforehand wich i am regretting, any ideas?

Nobody has seen this before?! Ive been google searching all day and have yet to find a pic where the yellowing is only at the tips up the top third of the plant...heat stress or light bleaching are kinda out as my grow cab is always 72 to 76 degrees and and the lights are at about 18 inches...

And now i remember why i dont use this forum anymore...

Well in case somebody else has this issue i'll post what i have concluded without the advice of an expert(so it may not be right), but im pretty sure im right...i have concluded that i waited too long to add flowering ferts and not enough of them, also the low ph was probably locking out at least some of what i added, and they started to show the beginings of a potash def. In the yellow tips, i noticed after the lights came on my stinky hippy pheno of chemdog had necrotic tissue running down the edges of one of the leaves, it had the least soil so i figure it got it worse than the others, also i have noticed a few light dustings of brown spots on a few leaves in the cab, probably just the low ph, locking out magnesium.

Since most were just watered the other day i cant do much for the ph today but i did have three that i hadnt watered yet so i gave them oyster shells on top of the soil and watered with ph 7.3, if they improve some ill have my answer, and making sure i correct the ph in all my pots will be step number 1, then, when im certain there is no lockout occuring i can give them their next dose of maxi bloom, if the brown spots get worse ill add a little magnesium sulfate as well.