Possible total suspended solids bounded by the Alum caught in the larger fish's gills, creating poor oxygen absorption since it's still malleable and sticky, which in turn cause the fish to belly up.
I can't see how this theory could hold as the "floc" is only VERY lightly, magnetically held together and is no "stickier" in clumps than it as was free floating particles and the "clumps" are easily broken apart back into the individual particles. If the suspended solids caused no oxygen problems before it flocked, I don't believe it could after since there is no chemistry change.
