Three 4" apollo sharks - Went in the big tank and less than 3 seconds two got eaten by my datnoids. Much smaller plecos and fish been introduced without this issue prior. 3rd one got moved to baby datnoid tank and became a victim of manslaughter overnight.
My stock list went from 40 FW down to 2. My worst should be when I tried for a red devil, a male giant gourami and a ray, though in different times. I was a lot more stupid, then.