I'd ask the people on the discus forum cuz i don't know too much about them. But isnt a 40 a little small for discus? Granted you could have maybe 2-3 in there but from what I know is they like to be in schools of at least 6 with 1 fish per 15g of water. As for the tire track eel it wont/shouldnt hurt the discus but it might stress them once it gets big. Just my .02 but ask the people on the discus part of the forum and they'll give you a more qualified answer!