I don't think nitrates of 40 and Ph of 8.4 is what is causing your deaths. People frequently let their nitrates go in FO, and my PH is stable at 8.4 and has been there since set up Jan 05. Are you having major PH swings? I know that can cause stress, but there is still an underlying issue you haven't caught yet. How are their appearances before dying(can clue you in to certain diseases), what are you feeding, salinity with refracto, and what are their tankmates? I just do NOT think those 2 water parameters are enough to decide a cause of death.