PH Crashed Damn what's going on Now!

PH crashed in my 33gal three days ago.
stats were ammo 0, Nitrite 0, nitrate 10
but PH crashed to 6.0, KH was 0, kaput, nonexistant.
GH was very very low like 10. My Pleco died.

Did 50% water, PH only went up to 6.5, KH was still really bad.
Waited 12 hours and added 1 teaspoon baking soda.
PH went back up to 7.5 KH was 60 and GH 80. Fish got much
happier.

Today fish are acting all funny like they are hurting again.
Some are sitting on the bottom, most are bunched
together and hanging out near the filter inlet. Two of them
have minute signs of fin rot or look a bit frayed on the tips
that seems new. I rechecked stats all seems normal.
PH 7.5 KH 50, GH 60, ammo 0, nitrite0 nitrate10.
Did another 30% water tonight and they seem much happier
again.

Tank is a bit overstocked but it's been running a long time
and never had these problems. I'm running a good airstone,
an Aquaclear300 with lavarock and biomax media, temp is
72, i use 48hour airstone aged temp matched water with
recomended shot of dechlor. I also run a 0.05% salt solution.

I usually do a 30% water
with gravel siphon once a week. However, now i really
really siphon the gravel, I run a garden hose to the basement
so it really gets the krud out. My Nitrates are really low now,
always less than 10ppm. Could it be i'm overcleaning the gravel
and killing off good bacteria? If so shouldn't I see ammo or Nitrite
spike? Did the PH swing cause long term damage that will take a
while to recover? Did the PH swing hurt my biofilter? If so wouldn't
I see a spike?

Anyone have any idea what's going on here?

....Kodiak