How should I clean a fishtank (+rocks+ornaments) for storage so that it will still be safe for new fish later?

Gidget Gannup asked: Unfortunately, my last guppy (Ernest) has died. I don’t want to get any new fish at the moment, just pack it up. Is there anything that won’t store (it’s got rocks and ornaments and fake plants)? As it won’t be reinhabited immediately, can I use anything stronger than plain [...]