No filtration should be needed. Just put a cheapo powerhead and a heater in the tub. Basically mimic tank conditions minus the light, don't want to grow algae.
And no, if it smells rotten just do a water change, or spray it off, then soak again for a week, this is just organics rotting away, renewing the filtration abilities of the rock.
I was more addressing if it smelled like chemicals. Some people try to bleach or chemically clean the rock, but don't follow thru with the clean up. Thus leaving you with toxic rock.
But judging by the color of the rock, it hasn't seen bleach.
If you soak, and it smells "earthy" you probably have a good bit of rock. If it smells "rotten" spray it off, soak some more. If it smells "chemically", cross that bridge when you get to it.