This sounds like a good one. Put it in fresh water and overdose the normal freshwater dechlorinator. This should do the trick. Good ones remove more than chlorine but metals and other staff too.Let it dry in the sun for a couple days, put it in a bucket with an airstone and a large helping of dechlorinator for a few more days. Rinse, sun dry, repeat. It took 3 cycles of this to get my rock to not smell like a public pool