The eel is a fish, correct? I think you'll have to remove it, then leave the tank empty of fish for at least a month; I'd wait longer. Then, think about what fish you ultimately want to have, meaning your complete population, and come with a plan for stocking them in the least stressful manner. (territorial fish last) Then, you have to take great care to buy healthy fish; captive bred or reared fish are much more disease resistant. Then, get a few cleaner shrimp, although if you want the trigger that might be problematic!

If you have a fish only set up you might consider a UV sterilizer, especially if you can't keep cleaners.

Good luck!!