Hi flemmyemmy 
If the bogwood can naturally lower your pH a little, it would be good for the corys. That doesn't address the problem you are having now though. PH doesn't just jump around like that. If you can find out the reason that it happened, you can probably prevent this from happening again in the future.
While you do have some nitrates in your tap water, you can work with it by doing larger and/or more frequent water changes than you would if it was at 0 ppm. It will help too if you avoid overstocking the tank.
Another thing to think about is the temperature of your water. C. sterbai are from warmer waters and need to be kept in the temperature range of 76-82 degrees F. They will not thrive if it is any lower than that.
If the bogwood can naturally lower your pH a little, it would be good for the corys. That doesn't address the problem you are having now though. PH doesn't just jump around like that. If you can find out the reason that it happened, you can probably prevent this from happening again in the future.
While you do have some nitrates in your tap water, you can work with it by doing larger and/or more frequent water changes than you would if it was at 0 ppm. It will help too if you avoid overstocking the tank.
Another thing to think about is the temperature of your water. C. sterbai are from warmer waters and need to be kept in the temperature range of 76-82 degrees F. They will not thrive if it is any lower than that.
