How would a great flood kill fish (a flood is water ... fish live in water) and embed little spherules of formerly-molten glass in their gills?
The Bible tells us how before the flood, it did not rain. A water canopy was in the atmosphere which shielded the earth from harmful rays (UV, etc) and there was a continuous “spring time”, which was why possible life span of people in those days was several hundred years. This water kept the earth moist enough with water vapor. The other half of earths water came from underground. When God suddenly unleashed both of these collections of water there was great upheaval caused by the tremendous forces. Not only did it alter the ground features, but most likely caused volcanic eruptions as the crust shifted. This could account for the “formally molten glass in the fish gills, and certainly could have caused fish in shallower water or in lakes and rivers to be tossed about and killed.