First a warning: do not fiddle with the pH
. Fluctuating pH can be highly stressful to fish, weakening the immune system and can even kill some species, though Jewels are pretty tough, but still the stress of this will have an impact.
To the issue. The pH is closely tied to the water hardness. Rather than detail all this, I will refer you to my article on the subject: http://www.tropicalfishkeeping.com/f...quarium-73276/
This will provide the whole story so you can understand the connection.
Next, the fish. This species, Hemichromis bimaculatus
will manage fine in waters with a pH from 6 to 7.8, so your pH at 7.6 is fine for this fish. It does not need to be adjusted.
Now to another issue: your tank being new is likely not cycled, hence the fish spazziness. Read more about this here: http://www.tropicalfishkeeping.com/f...m-cycle-38617/
No mention is made of the tank size, but this fish attains 6 inches and is a rough customer. It needs space to grow properly, and will require a 4-foot tank.