Hydrogen sulfide, oxygen, pH

I have a 75 gal planted tank. After changing the
UV bulb in the sterilizer fed by one of the
filters, I forgot to open one of the four valves
and left it like that for about three weeks. When
I was under the tank looking at something else, I
noticed that it was closed and stupidly opened it.
The smell of rotten eggs, I suppose hydrogen
sulfide, quickly filled the tank. I immediately
did a 75% water change and completely changed the
media but by the next morning fish were gasping
for oxygen and the pH was low even though the
water no longer smelled. I turned off the CO2
injection (it does not run at night anyway) and
pulled a filter return out of the water so it
would aerate and did this for the next two weeks.

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