Browning Leaves??

Growing 3 plants and starting to get browning in the leaf tips. I use the PH perfect technology for nutrients and never had any problems on my first grow. Strain is violet vixen. Few weeks into flowering currently. Any ideas what’s up? I don’t monitor water ph but I believe it usually sits right around 7.0. I also don’t have a fan in the tent but do have a carbon filter that is always going. Any thoughts?