Curious, white stringy poo is pretty much internal parasites, which in turn, means that the eel has them now. It can literally take several weeks for an infection like this to progress to death in an eel. A proactive response would be the best approach even though you do have time. Treatment is always easiest when the parasite load isn't as large. What will happen is the parasite load will increase to the point in which it will interfere with his appetite, then they will stop eating. After that they will get a very voracious appetite, basically starving to death due to the parasites consuming everything, then they will stop eating for the last time. This can take a while; because eels have a reputation for going on hunger strikes, many people miss this and in turn it ends badly. Lions don't eat poop therefore they can't catch the internal parasites from the eel, but other fish many times will graze on the poo which in turn means they can catch it. Lions can tolerate the treatment that is needed, which would be general cure or fritz paracleanse. These 2 contain both prazi pro and metro which covers the widest range of worms and internal parasites. While prazi pro alone may work, depending on the type of parasite, metro may be needed. As stated above, be very careful of dosage, again to the water volume not tank size. I keep repeating this because I have found so many cases of failure, which almost always points to an overdose.