What kinds of things are proven to cause severe PH loss?

Basically I have two systems that are identical in same environment just have
two different strains in them.

The system with blueberry the PH is solid as a rock and I buffer at the beginning
of the week and it stays there all week long.

My sweet island skunk/sour d system will lose .5-.7ph within a 24 hour period
which seems ridiculous. While I'm constantly re buffering it to the right level
it's constantly always dropping.

What kinds of things have people found to cause severe PH swings downward in
DWC? Can certain genetics simply cause the PH to go out of range from their eating?

If they are drinking more than they are feeding the pH can drop (as the solution ratio between water/nutrients changes). Is there a big difference in your two systems as far as needing to top-off water? Anything that might cause the skunk/sour plant(s) to transpire more (like temp. or air circulation differences)?

Dead plant material in the reservoir or in the root mass? Are the roots healthy?

If everything looks great with the plant, I'd suspect it's related to the ratio of drinking vs. feeding.

If you are feeling experimental, you could reduce the amount of nutrients (ppm) given to your skunk/sour and see if that tends to have a stabilizing effect on the pH.

I appreciate the input.

I use hygrozyme/great white/zone in conjunction with 61* res temp which
keeps my roots very clean. Between the two systems the SourD/SIS seems
to drink faster than the BB. How much ppm shift in a 24 hour period is too
much? I have a calander setup where I keep daily tabs on the ppm/ph
and there usually isn't more than a 20-30ppm shift in the nutrient solution
in a 24 hour period. Could that bee too much? The BB seems to have less
shift, but I figured that was simply because it was a single strain in the system
and I had better dialed it.

I'm on week 6 from clone (1 week flower) and I'm only feeding them about 650 atm.
I can try and drop it down a bit more, but I might try keeping it at this level and see
if they catch up and things get better.

I did notice one trend that the first 3 or 4 days things go relatively ok, then all the sudden
that's when the drops start happening rather quickly

Yeah if the sour D is drinking more that would make sense with seeing the pH in the reservoir drop.

It's funny, for the longest time my plants weren't drinking much - I would have to top-off maybe once a week. Then all of a sudden I was replacing about 2 gallons every few days and having pH issues (dropping pH)... then all of a sudden they would go back to hardly drinking and pH would be rock steady through the week. Temperature/nutrients/lighting was staying about the same so it was baffling me. And just the other day, I noticed the shifts coincided with me keeping a window open or closed in the room when the weather was nice or not. Apparently, the shift in humidity from outside air vs. A/C air was changing the transpiration rate of the plants which was the cause of them drinking more or less which was effecting the pH... I just mention this because there are so many damn things that can effect the plants - I mean, who would have suspected a window being open or closed as being responsible for pH problems?

In any case, different plants and strains will tend to drink (and transpire) more - or less. Different plants and strains will tend to uptake nutrients more - or less. Environmental conditions can impact both things... so it can be a difficult thing to dial in especially with multiple plants and strains. Wish I could help you out more but you'll likely just have to go experimental on them and keep trying things until you find what works for that particular plant if you are tired of chasing pH around.

It's just a very general rule of thumb that if pH is dropping in a healthy system with a healthy plant, it's because the plant happens to be drinking/transpiring more than it's feeding... Which means you can either drop the nutrient concentration to minimize the pH shift (while still topping off frequently) or upgrade to a larger reservoir to minimize the pH shift (and not have to top off as frequently).

how often are you doing res changes? this can also cause ph swings.