Based on your description they're NOT planaria. Planaria are free-living 'flatworms' and are usually seen cruising slowly along the substrate, decorations or aquarium glass. What you're describing are likely free-living nematodes (roundworms) which often exhibit a 'wriggling' motion as they swim from place to place.
Nematodes are relatively common in freshwater aquaria though they're not usually noticed unless there is a population explosion due to a build-up of uneaten organic material in the substrate. It is also very possible that you brought in a healthy crop of them with your 'coontail'. The reason they have disappeared is probably because they've taken up residence in your aquarium substrate or have settled back into the 'coontail' again.
It is important to note that, while planaria are basically harmless, there are many species of parasitic nematodes (including some that are serious parasites of humans). Chances are the ones you've noticed are harmless free-living varieties but it would be in your best interest if you keep an eye on your fish for any signs of listlessness, decreased appetite or weight loss. A thorough substrate vacuuming wouldn't be a bad idea.
-Joe
Nematodes are relatively common in freshwater aquaria though they're not usually noticed unless there is a population explosion due to a build-up of uneaten organic material in the substrate. It is also very possible that you brought in a healthy crop of them with your 'coontail'. The reason they have disappeared is probably because they've taken up residence in your aquarium substrate or have settled back into the 'coontail' again.
It is important to note that, while planaria are basically harmless, there are many species of parasitic nematodes (including some that are serious parasites of humans). Chances are the ones you've noticed are harmless free-living varieties but it would be in your best interest if you keep an eye on your fish for any signs of listlessness, decreased appetite or weight loss. A thorough substrate vacuuming wouldn't be a bad idea.
-Joe
