pH 7.6? Short of RO/DI water conditioned with whatever, if you are using tap water, you are looking at peat, a water-softener pillow in the filter (ion-exchange), or a pH-Down product which is based on hydrochloric, sulphuric acid, or bisuphate salts, but they will only temporarily work.
Absolute IDEAL conditions tend to implicate very costly measures, the likes of which most of us can't afford to pursue, especially if you insist on using tap water, and it is hard.
Eliminate tank contents as the culprit first (collected rocks; any shells;etc.).
If the tank is planted, CO2 injection with electronic controllers/bubble counters are the way to go by controlling CO2 input, otherwise you are limited in your options.
Phoshate buffers cause a multitude of problems.
Many peeps think that that they can manipulate ideal CO2 conditions by manipulating pH and KH by chemical means, but it just does NOT work, no matter what product you use.
I could elaborate on the science, but better yet, take my word for it.