sad day

[Scythanith]

If the systems aren't tied together I'd have to guess it would have been a common factor to cause all of them to die, i.e. heat, or food source.

Heat is easy to understand. The food source could have been contaminated OR the hands of those who fed could have had something on them that cause all the death. Aside from that it could have been a scrub pad or some implement shared between all the tanks.

Sorry for your loss, that's all that really matters!