Plants should be fine. I don't think your trap will work though--not if they're bloodsuckers. They'll need live prey. Actually, if they do end up in your trap, it will mean that they won't hurt you or your fish and you can keep them!
Would it be possible to get a really good photo of one? The video sure moved like a leech and was shaped like a leech, but without a closeup, it's possible they are something else altogether.
With as many leeches as you have, I'm sure the goldfish would show damage if they were being attacked. Come on...capture one...fast it a few days...introduce snail.
And yet...gross. You're still gonna want them gone!
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