Sorry for your loss, I have heard of this happening from time to time from using hot water to make the water warm. Water heaters are a major source of toxins.
Here is a quote from a member on another site from quite a while ago;
"You should not do water changes straight from the tap this time of year. Cold water holds more soluble gasses, and as it warms up in the tank, they are released from solution. Some of those gasses are toxic, others simply result in gas bubble embolisms. Large water changes, such as those 50% changes you've been doing, are very risky at this time. And you should NEVER mix hot water from the tap for water changes, as water heaters are a major source of toxins. When the cold water gets colder in the winter, you add more hot water to compensate, and a higher percentage of toxins are introduced to the tank. Many "mystery" deaths can be linked to these causes after the fact.
This information came second hand, from two people who have been among the greatest fish breeders in the country, James K. Langhammer and Rosario LaCorte. Both checked these facts with chemists, and have been passing them on to those who read club magazines or attend conferences for the last 30 years or so."
Take it for what its worth too you, but I NEVER use hot water to make water warm for my tank after reading this a few years ago. I set my water out in 5 Gal buckets the day before to get to room temp.