I am glad your CUC is doing well.
As for the question about the PH buffer.... I have never had a PH swing like that... But I asked about buffer when I first got into the hobby and I was told that it will only mask the problem and that if your PH were to take a bigger dip, you wouldn't know it was as bad as it was, because of the buffer.
As to what you would use, you would have to ask the experts. I have never had a chance to use a buffer, and for that I am happy. But I would think there would be something else you could do or use... ? water changes? I don't know?
Anybody have an answer for this?
Have you gotten your pods or live rock yet??? make sure you do that 1 month before you get your mandarin