Could have been just nematode worms which get into plants and gravel and mud and basically are all around us (ahhhhh!)--relatives of microworms and vinegar eelsl, but without a picture it's hard to be sure. Many nematodes are harmless and generally just take dead matter back to an earthy state (sort of like earthworms do). But, they could have been planaria (also supposedly harmless although planaria will eat eggs and multiply like crazy in a tank). Could have been leeches. If you're going to take plants out of a pond, you need to sterilize them before putting in an aquarium. I use Potassium Permanganate (pond stores sell it I think), turns the water purple but kills snails and other plant hitchhikers. You can also do a thinned down bleach rinse although some plants are too fragile to handle it (something like 2 parts bleach to 98 parts water--not real sure about the numbers). You can also do an aluminum sulfate bath which I've never tried, yet.
