Apologies if I came off as rude, just trying to figure out what the source of the problem could be.
If your tank is 3 months, and you do regular WCs, I don't think a bit of overstocking could be the source of the deaths. Could you specify, where did the nitrates come from that gave you the high reading? That is prob what killed them.
Also, how is the tank doing now?
May I ask what salt mix you're using? Do you know the alkalinity of your tank? If it went for a long time without a wc
, the dropping alkalinity is probably what caused your significant ph drop.