The worst would be a black volitain, it went from a 20dollar fish to a 200fish in about a month, ate all my inverts, and fish, and was a pain in the @ss... Second worst would be Two, count em', yes I said, as in 2-snowflake eels in a 29gallon.... tall! I did it but never again. They were happy to move to the 125, but I read some books, asked around, and decided to trade em in for store credit.