serious wilting in bubble buckets

I nursed a train wreck from a tiny clone that was a throw away to a beautiful plant. My brother and I then put it in a big bubbler bucket since it had gotten big enough. Now it along with a few others, have suddenly developed hanging droopy leaves that are yellow then turn crispy and die.
I think it is nutrient lockout since when the buckets need topping, bro said to add more nutrient water. I am new to bubbleponics so have been doing as he said, but his plants grow for awhile and then develop this problem and die off. I do not want to allow my plants to die off like that and prefer to try and save them. So here is the information as I know it, please correct anything you see wrong since I am moving my plants out of his room and into my closet to be taken care of properly.

Ph found it to be very very low around 4.5..he says 6.5 but my research says it should be in the 5 range.

Light..not sure about wattage but the plants are not getting burned and grow very well until they suddenly do the droop thing

we are using flora grow (both parts)

roots look great: white and some as big as my pinky.

I use nettle tea as both a foliar spray and mite controller. I think topping buckets with nutrient water is creating the problem since the train wreck does look better since i topped off the bucket with Ph'd tap water and used the foliar spray on it.

Sorry for the novel lol...any help would be appreciated

Could be a combination of things but heat stress on the roots seem to be lighting a bulb in my head. Whats the temp of the water in the bucket? if it exceeds 78 degrees F add a frozen liter bottle to the bucket.
As for the ph, this is how I do mine. Once a week, empty you bucket, fill it with new water, add your nutrients, wait an hour for it all to mix, test the ph, adjust if needed, and test and adjust everyday, once a day if needed. Your ph should be 6.0 but anywhere from 5.6-6.5 is tolerable but not preferred.

Update
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Do you have slime on your roots? Feel them

roots feel fine not slimy and since it is starting to be chillier here I am pretty sure heat is not the issue. In fact I may need to put a water heater sleeve on it soon lol. Though once I move it out to isolation I will keep an eye on the water temp since it will be in a smaller room. It is very sudden nearly overnight and seems to work its way from top to bottom.

Problem could be poor oxygen to the roots... Common problem with DWC system. It could also be root rot/ root slime/ and plant herpes. Yes, I mean herpes, superherpes

herpes! oh just my luck how is this treated? what should I watch for? the roots are bright white and not slimy though I did get a second air stone for it. It has had an air stone this whole time, but I am willing to add more. I will try and get some pictures up somehow though I have no camera.