Sometimes they work, but if they work it's because of the live bacteria, which could be dead of exposure to extreme temperatures or the product is too old or expired which would render them useless.
Edit: If I was gonna try it I think I'd use it to jump start a fishless cycle, so if it doesn't work, you've not really lost anything. If it does work, it will cycle the ammonia and nitrites quickly and you can add fish, will have taken a couple extra days but you won't be risking the fish.
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