I agree with Doug that you need water movement and a heater. I'd also add a light for several hours a day, too, to keep the coralline growing or at least alive. I wouldn't change the water until it's cured or near ready to go in the tank. How long to cure? Could be days, could be weeks, depending on how much dead stuff you have on it.
I've got 25 lbs of cured LR in a rubbermaid container in the living room. After a week of stench, I took a bucket of changewater from a recent water change and scrubbed each rock in the bucket with a toothbrush. Man, that water was
BLACK when I was done
Within a week, the rock was cured.
I can hardly wait for our new tanks, stands and lighting
