What's wrong with my plants? Dark spots and yellow leaves

[mouhmad]

these dark spots started showing up and the leaves are turning yellow and dying. Any idea what it might be either cal mag, salt lockout, magnesium, manganese im just all over the place.
Temps 76-78 lights on 65-68 lights off
Nutes are tiger bloom 2 tsp big bloom 1 tbsp
Growing medium- promix
12/12
3 ft from light and finishing week 4 of flower with really nice buds it's just the lower/mid fan leaves atm
19 plants in 5 gal pots
Haven't checked ph yet which I think might be the problem but would I need to make up for it with a supplement or just flush and check ph from now on?
I feed every other watering (once a week)
about to finish week 4 of flower

[St.buds]

You gota stay on top of your ph and it could even be nute burn but I'm not sure at least this will bump up your post

[Canuck147]

What kind of lights? Your plants would be much healthier had you pH'ed your water from the beginning. Certainly won't hurt to begin.

[mouhmad]

hps lights 2000 watts i dont think its nute burn since i space out the feedings and played it safe a few times and skipped the nutrients

[Flockshot]

Start with PH.