Was the rock dry or was it covered in water? If it was only completely dry then it really wasnt live all the BB would have been dead. If you put the uncured rock in you new sw
then all the die off is what would be causing you ammonia and nitrates. IMHO
I wouldn't of put the clowns in. Your tank sounds like it is recycling or going through a mini cycle. Also the dechlorinator only removes small amounts of ammonia. Tap water is just a bad idea, no matter what chemicals you use. If you don't have another sw
tank like a qt to put the fish in, then they may be under quite a bit if stress with all of the fluctuations in the water. Can you give more detail of how all the condition of the rock and tank when you got it.