Goldfish are bad for any predatory fish, even if you raised the goldfish yourself. Goldfish have thiaminase which prevent fish from being able to process vitamin B1 leading to thiamine deficiency syndrome.
http://www.wetwebmedia.com/ca/volume_6/volume_6_1/thiaminase.htm
Thiamine Deficiency Syndrome and its symptoms
Vitamin B1 (also known as thiamine, thiamine hydrochloride, and in older text books, as aneurine hydrochloride) is an essential nutrient for most animals. It is a colorless and water soluble chemical that helps to convert carbohydrates into glucose. It is particularly important for the correct functioning of the nervous system. A lack of Vitamin B1 is called a Thiamine Deficiency Syndrome.
Symptoms of this syndrome are well known from several commercially important fish groups and can be confirmed using appropriate biochemical tests. Flatfish fed exclusively with thiaminase-rich clams suffer and die from paralysis and related physical shocks. Eels show a trunk-winding syndrome and hemorrhages along the base of the fins (similar symptoms have been reported from moray husbandry, too). Salmonids show nervous disorders, poor appetite, poor growth and jumpiness (again, similar things have observed among a variety of ornamental fish species). Skin congestion and haemorrhage have been reported from carp and other cyprinids. In general then, excessive amounts of thiaminase are connected with symptoms of sickness that include poor growth, loss of appetite, abdominal swelling and hemorrhage, loss of equilibrum, convulsions, muscle atrophy and a weak immune system.
