Not a myth. It's sometimes hard to avoid. I had a shy, slow moving discus that has scars from my pleco. I used to think it was the otos, but after removing them (otos), the marks still appeared on my discus. It still happened, no matter how well I fed my plecos (L-018 Gold Nugget & L-260 Queen Arabesque). I eventually gave the discus away to someone else. Unfortunately, it started happening again when a pair of started spawning. The pair stayed to one corner. And when discus spawn, they start excreting extra slime coat, which my pleco got a taste of. I eventually removed the pair and placed them in their own tank. It hasn't happened to any of my other discus.