Does it cycle the tank? Or like, start up the process? Make it go faster
Old decor + old substrate + filter media + new tank + new water=cycled tank
IF the old stuff stayed wet. If dried or overly rinsed, the bacteria died and it makes no difference.
Most the bacteria is in the filter media and substrate, but certainly there is lots on decorations (especially driftwood and porous rock)
Old decor + new tank=seeded aquarium=shorter cycle period (again only if it stayed wet and fed)
Also, if doing a fishless cycle you can speed it up by turning up the heat and other tricks in addition to "seeding" from a cycled aquarium
