yellow and brown dry tips...help if u can....

I read a few things here and there but I would like someone elses feedback in what they think this is...this not a topic I can ask just anyone around me...this is week 6 on my auto I have autopounder and hijak and whitewidow all fem all autos under 600 hps at 18/6 my temp varies from the high 80's and low 90's...I have them in 5 gallon pots since germination on (ffof) and feeding them fox trio pack at half the dose...there water is distilled or bottled water never tap at ph 6.0 or 6.5...now this is my 3rd week that I noticed this problem still happening...I don't know if I should flush them with ph water or give them Epsom salt or cal mag...this is my very first grow so I just don't want to jump the gun and try all sorts of things and make it worse or maybe I should not worry at all...now there has been times that I do not feed them or water them for 5 days also so I don't know if that can also be the problem...if anyone has any input please don't be shy...oh yeah I also have a 600 mh on other side of tent for my lil guys and on one plant the same problem has started also...here are some pics I hope they are helpful so you may help me...View attachment 2849092View attachment 2849084View attachment 2849088

Definitely Phosphorus Deficiency :
Phosphorus aids in root growth and overall health of the plant and is especially essential when the plant is in flowering. Cannabis loves phosphorus and it is very unlikely that cannabis gets too much phosphorus. A plant which is deficient in phosphorus will have slow, stunted growth. If there is a major deficiency, the leaves may exhibit dark gray or purplish splotches. The last picture below is an info-graphic with more information about phosphorus and your marijuana plant.
What you have to do is check the pH and flush system. An excess of Fe and Zn may cause a phosphorus deficiency. If you've tried everything else, then you may try adding a bit more phosphorus to your feeding schedule and see if that helps clear up the problem. Cannabis plants love phosphorus, and therefore it is unlikely that you will give your cannabis too much phosphorus.

Thanks for the reply, will definitely head to the hydroponic store tomorrow and pickup some calmag is there any particular Epsom salt I can use for my plants? And what can I add to increase my phosphorus count on my plants? Today I kinda flushed my plants with ph water I was suppose to feed today but decided to give them a drink instead until I get the stuff I need for them...