[Huma Huma65651]

I got a fish for my aquarium a couple days ago and my ph was 8.2. Now its between 7.8 and 8.0. I did move one rock around my aquarium. I also changed the cartridges in my filter. I put some chemicals in my aquairum too. Could this be the reason? Also my nitrates are 2 and my ammonia is .1.

[leebca]

It seems like the aquarium wasn't ready for any additional bio-load to be placed in it.

Besides overstocking, a sudden surge in biological wastes and activity(bio-load), bad saltwater mix or source water or both, making the wrong additions to the tank, there are several technical reasons for a lowered/lowering pH in an aquarium. Nothing explains it better nor more thoroughly than this reference article:
Low pH: Causes and Cures by Randy Holmes-Farley - Reefkeeping.com

[Huma Huma65651]

My tank is around 6 months old now. I thought you could add fish in around a month.

[leebca]

The preparation of a marine aquarium for fish is a long process, usually starting about 6 months before the first fish is added.

The process is covered here:
Setting Up a FOWLR Aquarium

Additional information about a mature aquarium is covered here:
The Mature Aquarium

It's not a matter of time, it's a matter of the proper bacteria and micro fauna and flora taking up residence and, being properly established, can handle shifts in the bio-load when those shifts are done slowly.

AFTER ammonia and nitrite readings remain absolutely 0 for several days, then the would-be aquarist begins to add life to the aquarium in the form of a clean up crew. This and the further process of introducing marine life to an aquarium is covered above. It is a step-wise procedure to guarantee/assure that the proper microbial lifeforms are healthy and standing by to do their job (which is to let the aquarist keep fish).