i would say don't worry. Especially if it is only happening towards the end of the plants life. Plants generally turn yellow towards hearvest time. The plant uses the food stored in those leaves as food. The leaves lose their color and fall off. Happens every time here.
^^^^ Depending on strain pH 7.0 may be too high causing a slight lock-out and the subsequent yellowing.
7.0 is great for flowering, but during veg it is better to keep it around 6.5 for optimal nutrient uptake. What fertilizers did you use throughout the plant's growth? What were your NPK ratios? Did you follow any grow guides while app nutrients?
to hear about the pest issue. Gnat larvae will eat roots down to the stem, and this can completely stunt an otherwise generous harvest. I have seen plants documented that had severe fungus gnat infestations, and the leaves were pale and discolored...even with balanced pH and NPK ratios. Peat moss is an excellent breeding ground for fungus gnats, and that is part of the reason I switched to coir fiber-based soils. The other way I've avoided fungus gnats is keeping an arid environment, except for the cloning area which offers no effective breeding medium for the gnat larvae to survive. Another thing to remember is that at the onset of 12/12 plants still need plenty of Nitrogen (some choose to give the highest ratios of N at this time) for the 2-3 week stretch. I keep Nitrogen of at least 1-5-5 (MetaNaturals Bloom) throughout the flowering stages until the final flush. I allow some yellowing leaves, but try to keep most of the large fan leaves at budding nodes dark green throughout flowering.
