I believe there are several types of seaweed available in grocery. When I was in Pune I did notice this. Try to find raw and unsalted if fresh is not available in your location.
Even frozen raw broccoli is a better diet than black worms for a Tang.
That said, malnutrition is not normally a cause of sudden death. However, a weakened state can contribute to a sudden death caused by PH shock, temperature shock, and/or salinity shock.