Losing vigor and yellowing

I wouldn't worry too much about small swings, its normal. Looking at those leaves makes me think its more than likely the environment.What light are you running,whats the wattage over what area and what are your temps and humidity.Tempos get too low for a lot of us at this time of year.They look very dry,too dry.Leaves will do that to consreve water.They say you can't over water coco and yours looks too dry.

PH of 5.8 seems off. I;d try and get that around 6.2-6.5 just give yea more wiggle room in the range of "ok" should it drift. No idea what your feeding them, organics/amendments or salts, kinda affects how you feed and water. With amendends you want little to no runoff and with salts you want about 10-15% pots volume in runoff to prevent buildup. Soilless holds onto less nutrients then soil does. Soil as act like a sponge and pack it on EC seems a bit high for me around 2.64 When measuring EC, you want to get a bit of runoff first then take a reading for runoff. Or mix all the nutrients up and then measure it in the solution and let it sit a bit Yellowing on the bottom typically means Nitrogen and some of the leaves look like they lacking Mg, but hard to tell without other information. I;d also wader that your watering habits are a bit off. https://420life.com/wp-content/uploads/2021/10/Marijuana-Leaf-Deficiency-Chart-scaled.jpg

Your runoff readings are fine, nothing that suggests heavy salt buildup in your media. If you're growing in soil it might be a little acidic but nothing too far outta range. Feeding at a little higher pH for a few feedings should pull it back up some. Really soil handles pH inconsistencies better than hydro options do. Your pots look super dry. Are you sure they're not just hungry? Might be waiting too long in-between feedings.

You need to keep your soil in good condition.I never check my PH and never have problems with it as I treat my soil.Always stand your water for at least 24hrs as the chlorine kills soil microbes.The microbes/bacteria/fungi will keep your PH in check.I brew compost tea at least once a month.I rip a chunk of soil and roots out of my pot then add great white shark/microbes and other things then brew for 24hrs but Ecothrive do an instant tea called Biosys which youy just water in.You should check out the compost tea and the benifits of soil microbes.If you're growing organic its something you have to know.

Your PH needs to be between 6-7. If that is not right, your guessing at what’s wrong.