Help, molly died

Hi, my three year old molly passed away yesterday, and I found her like this with a white string hanging out of her. Is this a type of worm?

Yup...that's a tapeworm

oh, no! How do I get rid of them in my tank. And what is my chance of getting them? Ugh.

Unfortunately, livebearers are susceptible to internal parasites and if one is infected then most probably all your mollies are infected. As this poor fish was already 3, its probably come from a newcomer you've introduced to the tank...you can buy dewormer from the LFS...for tapeworms, I believe its Praziquantel that you're looking for as the active ingredient. Hopefully someone else can be of more help who has more knowledge around the subject

I hadn't introduced anything new for like 6 months. I have 2 danios that are in that 20 gallon, that's it. I'll treat them and my other tank that shares a siphon to clean the tanks.