pH 16g

[KeepOnReefin]

I have a well established 29g mixed reef. I rarely have to add a buffer to rise/maintain the pH.... On the other hand I have a 16g that seems to be having water issues. I can't maintain the pH at 8.2-8.3... The tank has been set up for close to a month. I add SeaChem marine buffer and the pH raises like it is suppose to. A couple days later it drops down to around 7.8.. Live rock and sand is in the tank.. Any ideas?