I have a very large tire track eel (about 14 in) and 3 huge oscars in my 135 tank and they don't bother each other at all. Of course my oscars are not typical oscars and don't seem to care about anything else in the tank and are very picky eaters. So far my two biggest oscars won't eat anything but ciclid pellets and trout chow. Once in awhile the biggest will eat freeze dried krill. The smallest oscar of the three is begining to be just as picky and my tire track eel mostly stays under rocks and waits for me to toss in night crawlers and shirmp for him to eat. I've never seen him be agressive to any of my smaller fish in the tank either, and I have cory cats and long fin rosy barbs in the tank as well.