I put one drop of flatworm exit in every coral bag I buy or trade for. That way by the time I get home, if there are flatworms, I will start to see them come out of the coral.
I only had 1 that actually had any flatworms, but I knew it would.
As far as a flatworm dip, flatworm exit is #1. Just one drop. And if your trip is 30 minutes or an hour, they will be OK.
There are other dips for other things too.