FWIW, here's the "why":
Cyprinids (members of the carp family...goldfish, koi, rosy reds, etc.) don't have the correct fatty acids to be nutritious to SW fish. Their flesh also contains high amounts of the enzyme thiaminase, which inhibits the uptake of thiamine (Vitamin B1). An extended diet of Cyprinids usually results in poor growth rate, disorders of the central nervous system, clamped fins, cessation of feeding, and you guessed it, an early death.
Your best bet is a good, varied diet of SW fish flesh, shrimp, crabs, bivalves, squid, etc.
If you feed krill, feed it sparingly, as it has its own set of issues if used too often, and has actually been linked to lockjaw due to mandibular malformation caused by excessive fluoride (in the shell) and a complete lack of Vitamin C. Krill flesh also contains varying amounts of thiaminase.
HTH