I don't really know why KH has come into this argument?
Co2 DOES NOT alter KH. There are indirect ways KH can be affected by Ph but that is normally by raising KH. With this meaning that the increased acidity caused by the CO2 may mean that another source of carbonate (crushed coral for example) will dissolve quicker than normal and RAISE the KH.
I personally have no idea of my KH nor am I worried about it and push my CO2 to it's max. I have a breeding colony of shrimps and corys within this tank!!! I used to have Rams before an incident involving my 3 year old son and washing up liquid and yes they were also breeding nicely!!!
It does sound like a CO2 problem but more likely is asphyxiation than a KH problem unless there is indeed another source of carbonate within the tank to react with the extra acidity in which case the KH will be going in the upward direction and dragging the Ph along with it. The OP clearly stated a drop in Ph so we can rule this out!!!
There are some very advanced members of this forum who inject very high CO2 at a KH of 0, yes 0 with no effects to their livestock. They do not raise the KH with buffers nor want to. It is the road to ruin for most people when they start tampering with parameter changing chemicals.
As Truckasaurus says above oxygen does not dissappear as CO2 is added. They are independent of each other. This is why many of us use a good bit of surface turbulence to get O into the water and then increase the injection more to compensate for CO2 loss and therefore maintain our 30ppm or so. He is also right that you cannot take that chart as anywhere near accurate for reading CO2. This is well known within planted as it can give the impression to some people they are at 70ppm when in fact the were well under 20!!! This is why we use reference solutions in drop checkers and/or watch our fish/plants for a reading of the level.
This sounds to me like a simple case of a sick fish or asphyxiation. the latter I would doubt very much with a DIY yeast system!!!
p.s. Baking Soda IS Bicarbinate of Soda. That is what the US call it. They call Baking powder.......Baking powder just as we do
With increase dissolved CO2, pH will drop. kH remains the same due to the presense of CO3.
AC