I have 3 fire eels and have raised two 7" or smaller and they are now 16" to 24". Also, I have an unknown number of discus (lost count). All three fire eels are not mean, but are pushy. They get along fabulously sharing pipes together and what not, and don't harass any one in the tank. Its more like having a huge mastiff puppy; happy to see you but bowling over everyone to come say hi and get fed. I have never kept the two together for this reason. When they're were smaller they would have been fine. However, once they hit 12" I know they would upset the discus. But, I will say that discus have very different personalities. Some of my discus would probably let the eels have it, but some of them would stress out and die. I spent too much money on my discus to have them hiding trying to get away from the disturbance in the tank. Just my experience though. Each tank set up and each fish's personality can be different.