Poisoned?

[kribensis12]

First I would not use nearly pure ro water even with sachem. I would use a ratio of mixed water from the faucet with ro. If you want a target tds of 100 (tds is not hardness) and it is 600 from faucet then mix 5 part ro and 1 part tap and skip the sachem stuff. Second fishes are very sensitive to rapid change in water chemistry and temp. I suspect every time you do a water change there is variance between the water in the tank and the water being added and if this variance is high it will severely stress the fish. Cockatoo are clear water fish and do not need super soft acidic water to breed. Worse most cockatoo you buy are actually tank raised and they are not use to the water from their native habitat. You can of course adjust them to water closer to their native habitat but it should be gradual.
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While it is of course possible something was on the wood (I normally soak it in a pail for a week in advance - you should not boil drift wood); I would hope that you would at least rinse the wood off before adding it to the tank. Btw i also have used zoomed wood.

I would argue that changing the pH slowly (which has happened in my main tank and they have fry) is not a big deal when it comes to pH. The pH in the wild changes constantly - especially when the rainy season comes.

The fish that died were in perfect health before being moved.

The fry I have put in as tests are doing really well right now - so I am hopeful whatever caused the deaths is gone/diluted.

 

[kribensis12]

Welp - it's been 5 days and the mollies seem as happy as a clam (wherever that saying come from).

My A. Cacatouides fry are still pretty microscopic - so I suspect I will leave them with mom for at least one more week, if not longer ( I don't want her to eat them).

I intend to do another large water change (for good measure) and then port the fry over and hope for the best - since I have no way to actually no what caused them to do die. Fingers crossed!