Chara has the undeniable smell to it. My next bet is what you said...Hornwort (AKA Coontail). As you can tell...I am not a good betting man. I don't know much about Coontail, but Votex gives good general advice regarding - identification before eradication.

Try this site...

https://aquaplant.tamu.edu/plant-identification/alphabetical-index/coontail/


Fish on!,
Noel