Vapona was used by reptile keepers for years before it was banned. I used it once or twice many years ago. I used a 1" square in a cricket tub and left it overnight - no water. Then removed and replaced in 6 days. Then again in another 6 days. It was the most non-intrusive and effective method I've ever come across.
It was banned in the EU (and in the UK) because it is dangerous to manufacture.