I just did a fishless cycle for a friend's 20 long in a mere 10 days but she's decided she doesn't want the tank back! Lucky me, huh! But I don't like the substrate and I want to change it. I'm concerned that changing the substrate will kick the tank into a new cycle though. Any thoughts? She does want the decor back. I will be keeping a few of the fake plants, the filter, and the heater. I'm assuming the best I can do is to keep the water and a few handfuls of the substrate?

Zambize

There is a good bit of bene bacteria in the substrate, but I doubt it will cause a full re-cycle. Possibly a mini cycle at most. I went from some hideous gravel to sand (granted I had mostly same decor) without any signs of a mini cycle.