I don't have a PH meter, just some test strip...

I like to give 20 hours light so they're is sone dark time unless you're using co2 24h light doesn't benefit. Here is a link to something similar to what I started with and I think it is what you're asking for. You can buy online or find them in most garden centres. ssgthbfhf 3 in1 PH Tester Soil Water Moisture Light Testing Meter Kit Indoor Outdoor Garden Farm Lawn Plant Flower Meter £4.00 Amazon.co.uk

Hey man, the very dark leaves that start turning yellow from bottom to top / leaftip to stem is a pretty clear indicator of a nitrogen toxicity. You should seriously consider a full flush of the soil and just watering plain water for a few days. Then if needed just apply small amounts of fertilizer and increase as needed. Keep in mind that in flowering stage the plant needs less Nitrogen (N) and more Phosphorous (P) as well as Potassium (K). As for the pH topic, as @GreenMachine mentioned above, pH meters bring the disadvantage that theyre not really that exact unless you calibrate them every now and then for acceptable readings. I have one just to make sure nothing is completely off, but I would recommend strips aswell. And since your plants are flowering right now, every tiny bit of stress is decreasing your yield, so I wouldnt change the light cycle anymore. For your next grow though, give your plants at least 4hrs of rest during the night (20/4 cycle). The plants not only exchange most of their oxygen for fresh CO2 during the night, but they also use the energy gained over the day to use for growth during the night - 24hrs without a break is possible but the cost is not worth the result :) keep in mind that the energy cost ist just not worth the extra few grams you might get in the end. - WOW that turned out to be a huge book I just wrote, but I hope I could help you mate. Bless up my man.