I had this same problem on my 10 gallon nano with 6 gallon fuge. I use a pinpoint pH monitor and dose b-ionic. My problem was that I was dosing equal parts of one and two (alkalinity and calcium, respectively). Calcium was getting sucked up and so in order to try and raise it I upped the dosage on BOTH parts one and two, which was a mistake. The result was increased alkalinity and a decreasing pH like you mention. I spoke a knowledgable tech at ESV (the company who makes b-ionic) and just in time.
I was just about to start adding a pH buffer, but once you understand what's going on that would be a mistake. My dropping pH was a result of a higher alkalinity. Normally, my pH would drop to 8.0 at night, and peak at about 8.25 at the end of the day. My alkalinity was at 3.0 meq/l or 8.4 dkh. As my alkalinity went up to 4.5 meq/l, my pH would not rise as much during the day from photosynthesis, and would drop overnight. ADDING A BUFFER HERE WOULD ALSO INCREASE YOUR ALKALINITY WHICH IS WHAT YOU DON'T WANT. As Beth said, the answer is kalkwasser. Kalkwasser will do what the the two part solutions do, while at the same time raising your pH to a better level, sometimes too high. If you do dose kalkwasser I suggest a pH monitor, as you need to keep an eye on it. They are about 100 bucks, but they make up for the price in convenience. With the kalk, I keep my two part b-ionic on hand to adjust the alkalinity to 3.0 meq/l and the calcium to ~ 450, then let the kalk maintain it. This seems like a lot but this darn 10 gallon is trickier than my 180 ever was!
If you do not go to the kalk, lay back on the kent's superbuffer until your alkalinity drops. Add enough to keep your alk around 3 or 3.5 meq/l. When your alkalinity drops your pH should come up. If this doesn't happen there is another reason for the low pH. Be careful with what you add, and ALWAYS KNOW WHAT EXACTLY YOU ARE ADDING BEFORE YOU ADD IT. READ WHAT EACH PRODUCT IS MADE UP OF. WATER CHANGES WITH RO WATER AND A GOOD SALT MIX IS ALWAYS A GOOD SOLUTION. DILUTION OF POLLUTION IS A GOOD SOLUTION. That saying goes something like that. Hope this helps.