I did another 75% water change two weeks later. A
week after that pH was up to 7.2 with aeration so
stuck the return back under water (I have soft
water). Left the CO2 off. The next morning the
pH was 6.4 and three rainbows had died and the
other rainbows were gasping (rainbows and SAEs
seemed most prone to oxygen deprivation - angels,
tetras, and catfish were fine.). Even with
aeration and no CO2, the pH drops during the night
to around 6.7 and rises to 7.2 during the day.
Before the incident, I kept the pH around 6.6 to 6.7
all the time and everything was fine, but now it seems
that 6.7 means too little oxygen. Is this still
the effects of the hydrogen sulfide? Would adding
carbon to one of the filters or adding some
aquarium chemical help?
turf
Sounds to me like you KH has dropped significantly. A swing between 6.7 &
7.2 would tend to indicate there is almost no buffer left. Suggest you add
some Sodium Bi-carb to stabilise it before you get a Ph Crash.
I.