Bleach is actually the best solution, If your tank is still filled with water you can add appropriate amount of bleach, the bottle will tell you how much to add per gallon. Then empty out as much of the water as you can, let whatever moisture is left evaporate, the bleach will completely evaporate with the water and will not be harmful to the fish.
If you have already emptied your tank then soak your rocks and decor in a bucket with bleach diluted with water, after 24 hours empty the bleach and set the decor on a towel to evaporate the rest of the moisture. Wipe down the tank its self with bleach water as well.
I know this works from experience, I bought a used tank and put my fish in it and they all died, I soaked it in bleach let it evaporate and restocked it and my fish are doing great a month later.