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.
