Bil,
You must not have used the "buffered alum" and your alkalinity must be pretty low. Your PH probably dropped drastically causing PH shock in your fish as your low alkalinity was not enough to buffer the change.

Am I right? I would advise using the buffered variety for anyone with low alkalinity. Do you know what your pH was before the treatment and then after. If it is lower and does not rebound you may have to add something to bring it back up.

Conversely where I live in northern Indiana the alkalinity is so high that I have applied several times the reccommend amount even with trout present with no adverse affects. The only thing my treatment killed was snails and good ridence to them!

If pigs could fly bacon would be harder to come by and there would be a lot of damaged trees.