Disease or Low PH?

[marting83]

Less than two weeks ago, for some unknown reason, my PH dropped from the 8.2 to 7.6 in a matter of 48 hours (my testing interval). A couple of days later, I noticed that my Royal Gramma was no longer eating. He is now lying, almost lifeless, between two rocks breathing lightly.
I do not see any physical symptoms that would indicate a disease. Is this particular species of fish known to be sensitive to PH? I have 3 other fish in the aquarium - 2 False Percula Clownfish and a Yellow Tail Blue Damsel. They remain healthy as do all of my inverts. One other item to consider: his favorite spot is an area behind a huge mass of Mushroom Coral. Do they sting? Could this be the cause?
As the Gramma was lying in a open area of the aquarium yesterday, I noticed an interesting reaction from my CBS. Normally the CBS will try, unsuccessfully, to grab anything he can swing at. However, on two different occasions, he started moving toward the Gramma. When the CBS was roughly three inches away from the Gramma, the CBS backed-up and then turned away both times. WHAT?!?!
Though I am not confident that I can save this fish, I would like to know what is wrong with him so that I can prevent it in the future.
SPECS:
Aquarium Size: 30 Gallons
Filtrations: Marineland Emporer 280 Powerfilter w/ Carbon and Biowheel, Marineland Seaclone 100 Protien Skimmer
Heater: Marineland Neptune 200W
Lighting 96W PC 50/50
Temp: 80.2F
SG: 1.023 (1.025 factoring temperature)
Ammonia: 0
Nitrite: 0
Nitrate: 10ppm
PH 8.2 (currently)
CRITTERS:
Fish: 2 Percula Clownfish, 1 Yellow-tail Blue Damsel, 1 Royal Gramma
Inverts: 2 Short Spine Urchins, 2 Skunk Cleaner Shrimp, 1 Coral Banded Shrimp, 12 Scarlet Hermit Crabs, 6 Blue Leg Hermit Crabs, 5 Astrea Turbo Snails, 3 Mexican Turbo Snails
Corals: Mushrooms, Star Polyps, Button Polyps
Aquarium is fed 2x daily - Omega One Marine Flakes
Royal Gramma is 8 months old.
Aquarium is 14 months old.
Last addition to the aquarium was 3 months ago - 2 urchins

 

[lion_crazz]

Well a pH drop that much is not good. Have you ever tested your alkalinity?

 

[marting83]

Yes, rarely. Since I use a carbonate buffer to control PH, it tends to follow PH directly. However, as of 2 minutes ago, Alkilinity is 11dKH or 3.8meq/l.
Also: I forgot to mention that I do use RO/DI water. I can see that topic appearing in the near future.

 

[beth]

Yes, a sudden pH can cause death, and if this problem with the fish occurred at the same time as the drop, and in the absence of other obvious symptoms, it could well be. Keep an eye on your other fish.