What's wrong with this picture?

33Gallon with Aquaclear 300, well filtered and cycled (over 1 year),
I do a 25% water/gravel siphon once a week with 48hr air stone aged
temp matched water with recommended dechlorinator. There are
seven 1"-2" Goldfish in the tank.

Temp 72degF
PH 6.7, Ammo 0.0, Nitrite 0.0, Nitrate 5.0, Salt 0.1%,
GH 60ppm
KH 20ppm

Last 2-3 days the fish seem fine, but when I approach to feed, they
go nuts and eat like crazy, a few minutes later they go ballistic
in the tank they fly at super high speed from one end of the tank
to the other, sometimes bumping into the glass, this even before
they finish consuming all the food. Then they all jumble up against
each other in one corner of the tank not far from the filter intake.

Are they injesting something toxic on the surface? It's not the
first time I see my fish fly around like that, but it seems to
indicate something is very wrong, All parameters look very good
except KH seems too low. I did notice PH was a bit low (6.7)
vs normal PH is about 7.0. Was my PH just about to crash?
there is almost no buffer (KH) left in the water. Is adding
baking soda a good way to jack the PH up and buff up KH?

I know i'm a bit overstocked, but I'm diligent on maintenance and
Nitrate is really low only 5ppm. The only thing is lately I removed
the heater for a few days, so temp hovered around 68degF vs 72degF
normal, so I put the heater back in tonight

Should i test for something else like phosphates, or iron, or?

....Kodiak