Depends how big the arch is and how big your tank is. I threw mine in after a week of toilet flush changes. I had about 1/3 rock in a 90. Everything was just fine and dandy. Just toss it in and check the PH every hour. Throw it in tomorrow morning and watch it with the PH meter. If the waters PH has not gone up in 12 hours or so, i would say your safe, however....... While water is fine, any shrimp that hide on/under may not be okay. I did not think about this when I threw my shrimp in. Quite a few died and I chalked it up to when I saw one sticking out of of bluespots mouth ( I always got the small shrimp), so, not sure if it was him or the PH that did them in. Seems all my shrimp lived after he was gone, but that was a year after the rock went in. I never thought about doing a test or to check them for shrimp safe-ness. Just a thought, though, everything else was fine.
FYI - Made a sealed concrete (eccept for the entrance/exit) goby sleeping cave and threw that right in the next day. Goby was used to sleeping in it and faired just fine.
I have no theoretical proof, but, I think saltwater binds, seals, locks up the PH to a degree. You figure, I put 1/3 of pure concrete rock in my 90, which according to my bucket stated that in 24 hours the PH would go up to 10/10.5 . I decided to play devils advocate since I have a lot of flowing water and figured the PH would bleed off, and, sure enough, not a .1 change in PH . Your mileage may vary, but, those were my results. I tossed the other 1/3 of rock in the next week, same cure time and same results.