ph control with oyster grit

[howie]

finally got around to buying a bag of oyster shells(bird shell grit) for chickens from the local pet store ($21 for 5kg)
most of the shells were 5mm or bigger, so washed them and used a hop bag.
put exactly 2 cups in one bag so i can compare the volume later.
2 identical 25l rum washes
3l bunderberg molasses/3kg raw sugar/5l dunder(not live)/dap/epsom/80gms bakers yeast
big aerate with paint mixer
yeast hydrated and trying to get out of the jar
into the fridge fermenter, controlled at 32C with inkbird.
and we're off, with only one of the washes with oyster shells, hung in the middle of the wash on a SS fishing line
OG 1082 PH 5.3 for both.
1) non oyster - 24hrs absolutely going off it's head, had to take the airlock lines out overnight. SG is 1072, PH is 4.62.
48hrs - still fairly active, airlock lines in ......................................... SG is 1036, PH is 4.32
2) oyster wash - 24hrs no bubbling at all through the airlock, must be escaping somewhere?
perhaps through the grommit where the SS wire is.
lift the lid and there is a frenzy of bubbles around the hop bag and lots of fizzing all over. SG is 1060, PH is 4.74
48hrs - still no action through the airlock but wash is fizzing away SG is 1020, PH is 4.80

so the expected has happened with the PH, but the SG readings are unexpected.
especially with the non oyster wash going nuts.
i might take one more reading tomorrow and if the PH continues to fall in the non-oyster wash, will it be ok to add some shells at ths stage of the proceedings?
the PH looks like it will drop to around 4.00 by tomorrow.