You can't have fish in a tank with any ammonia. Whether the ammonia is from the cat pee or the dead rock or the chromis, it's still very toxic for the fish. It burns their gills when they breathe the water. I'm stumped as to what the best way to fix that situation is though, since you don't say how high the ammonia actually is and I have no idea how long it might continue to be released from the pee soaked substrate.
Honestly I would return the fish, throw out the substrate, read the sticky that mcgyver linked up-thread and start over. I don't want to discourage you but I think you've gotten off on such a wrong start that it really would be easier to start fresh than it would be to fix this. Just a few things that jump out from your posts, not to beat you up but to illustrate what would be involved in fixing the tank:
- Crushed coral is not the easiest substrate for a a saltwater tank, it needs special maintenance to keep from escalating nutrients. Idk what dredged shells are but I doubt they will help that situation
- Trace elements are wholly unnecessary at this stage
- Stability is a bacteria culture, not a water conditioner. It did nothing to render the tap water safe
- lava rocks in a canister filter is not a great idea
- canister filters often don't really do salt water tanks any good
- the stuff you are saying in the first post about "chloride" fixing levels doesn't make any sense
- hob filters are for freshwater tanks
I don't know if your getting bad advice from someone or just getting a little too creative trying to avoid replacing the pee substrate, but you are not setting yourself up for a good experience with the hobby or a happy life for your pets. Again, I hope that's not too harsh there is just really a lot of red flags here. Slowing down and reading at least the really basic parts of the linked thread where it describes setting up and cycling a tank will save you trouble and a lot of money that you are spending on unnecessary equipment and chemicals. Good luck
