Marshll,

First I feel for you. You are going to have to get more information. You
either have a water condition going on or you have a disease. Start by
checking all of your parameters. make sure ammonia and NitrIte are under
control. Check salinity and temperatore.

If those look OK then report back more information. What happened before the
first fish died. Did you see bulging eyes (popeye) Do you see what looks
like salt granules on the fish (Ich)

This is going to be hard because we cannot see it first hand and are reliant
on what you see. If you have a reliable LFS, talk with them. You can show
them the dead fish first hand and they may be able to help.