needing help

This is my second cycle with my little grow box and the first went pretty well but this time my plants are not doing as well as before. The plants are growing but they leafs are turning brown and brittle. I am using the Fox Farm nutrients and they worked great the first time and I am following the schedule that came with them. Any help would be greatly appreciated!!!

what is the ph?

The strain you're growing this time may be more sensitive to the soil. You may need to blend the soil with other medium.

Keep the ph at 5.8

its a hydroponic system

but the ph has been running high around 6.3 to 6.5, having to bring it back down

keeping the PH at 6.3-6.5 for a prolonged period can cause some nutrients to be locked out and cause severe damage. one thing with plants is that they need ALL the elements that are in NPK and all microelements to thrive. if you cut one of even the smallest microelements that they require in very little amount like boron manganese or so, the plant will suffer greatly. I like to get the Ph to 5.5-5.7 and let it drift up, i check in few days, if plants are drinking and eating, PH normally rises, sometimes even to 6.5 in 2 days then I lower it again, on average every 3 days. you can look into the charts that tell you which nutrients will be locked out and at what PH ranges, this way you will have a clear understanding why 5.8 is optimal

What is your ec/tds? And yes try to get ph down. I am also a complete newb but i run my system at 600-650 ppm's and 5.8-6.3 depending on stage. It stays pretty constant but at the end of my seven day refill cycle ph will start to drop. More so in mid to late flower.

I have had best results with a product called drip clean!
IF your EC is too high from your runoff you could prolly use it.
I stick to 5.8-6.1 however my plants are huge right now and are eating 1.5-1.9 EC Lucas formula with only FNB.
I have never seen plants do so well from such a simple diet!!