[aussiefishy]

just wondering if you have check your pH, or alkalinity.

first, i do not think the reason for your losses came from change of salt, skimmer not running or air pump not running.

the most probable cause would be poison, such as from excess use of impure chemicals, like dechorinator, or the elavated Mg might kills invertibraes or other micro-organisms that created a NH3 spike.

Can you give us a brief description of the fishes that were dead? like signs of physical trauma, or certain notable conditions?

you mentioned O2 depletion, did you check this with a reliable test kit?


recommended improvement might be regular testing of water parameters, keep log of these data and create a record of different happenings for your tank. From my experience, i did solved a lot of my tank's problems by checking these valuable logs that i kept over the years.

hope it helps.