Sounds like Threadworms. There is another name but it escapes me at the moment. Depending on exactly which species you have Clout will kill some of them. Some fish like Gouramies will eat them. If it's what I'm thinking of they are harmless, just ugly.

Lisa

New plants usually have a good amount of snail eggs on them, pond snails. When snails are babies they are almost transparent. They are kinda football shaped and when they mature the shell is more visible, and they look more "snail-like". They are harmless also. The more "food" you have for them, the more they multiply. Sometimes it is an infestation. Still harmless. Keep your feeding and algae growth to a minimum and the population will decrease. It is a part of having live plants. Well that is if that is what they are. A magnifying glass will help ID them. But if you just added new plants withing a few days to a week ago snails is a good possibility. I got clown loaches to eliminate mine. I didn't mind them but they grossed out my wife a bit. Another trick is sink a wilted piece of romaine lettuce in the aquarium, you can hold it down with a rock. The next morning pull out the lettuce with the attached snails.

Dobesrule/woesme: definately not snails, lol, more a worm and attach to walls, or moving through water.. looked up threadworm (aquatic) and closer... well guess one needs a microscope to make sure... at any rate seemed contained to one betta container, hopefully. Thanks both, Sherry