Major tank problem--ideas?

[Doglover_50]

Also posted on FRAG

I decided to try adding a yellow tang (post 30-day QT) into my 70g tank with my purple tang (no parental corrections please--I knew the risk). Anyhow, I'm about to buy a larger tank, have tons of rock for hiding, was all ready to pull rocks and extract the yellow tang if there were problems....last night, the purple tang was definitely ****ed, but I coaxed the yellow tang into a good hiding area, turned off the light to distract everyone, for almost 2 hours no fighting.

Then I woke up this morning. Holy cr*p.
1) Yellow tang is dead. No sign of cuts, etc...
2) My flame angel is dead
3) My royal gramma looked dead--wedged tight in a rock--but he is still breathing--for the moment.
4) Of my 3 skunk cleaners, only one is out and about--another one died, can't find #3, and am presuming he died.

Left living are the purple tang (freaked out all morning, looking better now), green mandarin, yellow goby, and cannot find the shrimp goby I introduced with the tang (he was tiny, hopefully just hiding).

So I'm wondering if something else may be contributing.

I had 15 gallons made up, did an immediate WC before even testing the water after finding the bodies this a.m.

Ammonia, nitrite, nitrates all look fine, alk looks normal, though my Ph is 7.8. It was around 8.0 last night, so it's low, but not a giant drop.

Any ideas? Could I/should I do another WC tonight? If so, how much on a 70g tank?

Thanks in advance for any thoughts...

 

[Doglover_50]

Oh--a few other abnormalities, in case they matter--this was all coming back from 10 day trip (I had a reef-tank savvy sitter) on Thursday, immediate water testing on all tanks looked good in terms of waste. Everyone was alive and looked healthy. Alk was down, I brought it up slowly over the day.

Also noticed yesterday my kenya tree had gotten sucked into the intake of my skimmer pump. I turned off skimmer and removed to a bucket to clean out the tissue. So the tank was without skimmer last night (oxygen depletion last night?). However, the tank has 5 powerheads to move water.

 

[bajabum]

If the kenya tree was decomposing, that could be the culprit, very toxic. 7.8 pH means CO2 went up. My have been even lower till lights came on. Adding alk means you added more CO2. I panic at pH 8.0. More CO2 means less O2. Lights out pH swings can be extreme when pH falls out from between 8.2 and 8.4.
Run Carbon.

 

[bajabum]

Confirmed with top dog at conference today. 7.8 pH and adding alk with skimmer off could have depleted O2. 7.8 is a critical point on the CO2 chemistry curve.