Hey DL thanks for the link to your show.
Looks like a case of heat to me also.
How is your rez temp ?
Lift the lid and take a picture of your roots.
Pen meters suck.
I have three dead pen meters
and two other dead meters.
a milwaukee sm101 and a Nutradip trimeter.
all dead..
Now I use the American Marine Pinpoint pH meter
which is more accurate
and has been pretty reliable so far.
They are the only one I know
that is made in the USA
and very accurate to .01
I get them on ebay for about $100
I have two
I am anal about knowing what my pH is
and I sometimes use both meters in the same rez,
just to make sure they are both on.
Pilots like redundancy...
I calibrate at every nute change,
since I always take my probe out of the rez
before replacing the RO and adding nutes.
While I am flushing
my probes sits in a vial of cal fluid 4.01
so I go ahead and do a full on calibration
If the temperature of my room has changed
since the last cal,
the meter will need to be "tweaked."
(that is an old electronics engineering term)
I saw from your early pictures of your clones
that you had the pH way too high.
But you got it down for a while,
so calibrating your meter is a must.
Can you get the room temp down into the mid 60ºs ?
I love being able to keep my room in the 60s
and even mid 50ºs
during the winter months.
The cool temps make those girls stand at attention.
I'll be watching your grow.
.