Before I start... DON'T ADD ANY MORE BUFFER!! This problem is much more common than many people would think... I have saved a link to an article that will definately help you regarding the whole ph/calcium/alkalinity mystery...
http://sps.reefkeepers.org/Alkalinity.html
Many reefers have a common misconception regarding dkh buffer, mostly because of the manufacturer's labels. Without getting involved in the chemistry, dkh buffer raises your alkalinity, which will raise your pH on a temporary basis. This meaning that the dkh buffer will bring your alkalinity to a safe level, which will aid in maintaining the pH levels overnight. If you have a very low dkh, say below 7 dkh, the buffer will raise that, which will indirectly aid in sustaining your pH when there is no photosynthesis going on.
However, when your alkalinity gets too high, say above 12 (this is a debatable range I realize, but I aim for 8 to 10 dkh), your pH fluctuates less and less, gradually lowering, and by adding more buffer you are raising the alk, which will lower the calcium and eventually the pH also.
One point I should mention is that it is very typical for pH to fluctuate up to .3 or even .4 in some healthy reefs, with the time right before the lights come on being the highest, and a few hours before the lights shut off being the lowest. Those with digital pH monitors can attest to that.
Beyond the pH monitor, what also helped me when I originally had this problem was using kalkwasser, and dripping overnight, which has the effect of slowing wanting to raise your pH overnight, cancelled out by the tendency of your tank's pH to LOWER at night. This results in both your calcium and alkalinity being maintained, and your pH staying somewhat stable through a 24 hour period. This is getting sort of complicated, and may be a lot to take in, but we can go back and forth if you have further questions.
First things first. Before you do anything. I ALWAYS recommend having your water tested at first by a second source AND second tester. In case you follow the steps wrong on a test kit, if you replace a test kit, yet still are going about the procedures wrong, you'll get faulty readings. Particularly on the calcium test, which is the most complicated with the most things that can go wrong. Find out exactly what your readings are from this source and compare. ALSO, test your pH in the morning and at night and list those readings. I am such a fan of pH monitors for this reason, so you avoid the "Is that 7.8 or 8.0... or maybe 8.2?" The difference between 7.8 and 8.0, and 8.0 and 8.2 is pretty huge... but not on that damn color scale!
Post the readings again after you do this...
7.7 is too low even at the low end in my opinion. If you want to go the additive route as opposed to the kalk route, you will need CALCIUM CHLORIDE on top of your buffer. If you use any of the two part solutions, usually part 1 is your dkh buffer and part 2 is your calcium chloride.
Lowering your dkh is the first thing to do. Do this by continuing with normal water changes, and not adding any additional buffer. With this alone, it HAS to lower. After that you need to get in a regimen of dosing calcium chloride AND your buffer, with different combinations being required for different tanks. WHATEVER YOU DO, DO IT GRADUALLY.
Lastly, I have a feeling your calcium levels are actually lower than 460, unless you've been adding calcium chloride... before I go on I'll wait for more feedback...
