auto-water change in a nano how much how often?

[kien]

For starters, if one of those timers or pumps die and you don't notice, the result would likely be catastrophic. Especially in a nano.

This approach is ok so long as you are there to observe the process and can intervene if something goes wrong. I certainly would not rely on it to just work automatically 100% of the time. And by Murphy's law, that ONE time that you're not there is when it will fail.