First off, I am soooo sorry to read this post. Dunno if the Alk dump caused the death of your fish, but I agree with what was said above - at the very least, it might have triggered this as a stressor to a pre-existing condition.
Actually, ATO/Doser/Controller failures such as yours are not as uncommon as many might think; and it's why I started an opensource project 4+ years ago to develop and build a Distributed High Availability Controller specifically to survive and fail safely from even hardware failures of the controller itself. In otherwords, if the controller were to glitch out while an ATO, Dosing, or Water Change operation were underway, a standby secondary controller jumps in and takes over, in very much the same way the Flight System computers on the Space Shuttle or CrewDragon would also handle such a failure. I also have 20 years experience designing and building such high availability architectures for international mobile telecommunications (aka GSM carriers) companies.
It's also based around RaspberryPi's, so it's quite affordable, flexible and scalable (from 1 to 20 aquariums and more) in ways mainstream consumer aquarium controllers aren't.
It does however, still requires some DIY and assembling the pieces and software yourself, but I hope to bring some Turn Key kits on offer later this year, for those who aren't technical, or don't want to assemble it themselves, and would rather just buy, plugin and go.
https://community.home-assistant.io/t/going-to-next-level-of-aquarium-automation-whos-with-me/18486
