Originally Posted by Jmlorfam
... and stability is based on proper maintenance & monitoring. ...
This really isn't an accurate statement, and that's probably why it doesn't make sense to you at this point.
"Stability" in a tank definitely won't happen if you are lazy on maintenance - that's for sure. But being on top of maintenance doesn't make the tank stable. It will help move the process along, but YOU don't have 100% control over the stability of a tank. Here's my take on it...
When you're starting up a tank, every new living thing you add to the tank effects the bioload of the tank. With that change, the whole system has to react and respond - additional bacteria will be produced, oxygen content will decrease and increase, and probably tons more things that we could never test. I like to think of it as a car with cruise control right after cruise control came out - when you went up a hill, you had anything but a consistent speed. You were either too fast or too slow. (Thankfully... better designs evolved and they're pretty good now!) After you're done adding livestock, your tank can finally go about trying to "stabilize".
What happens after a year? Nothing, really. It probably just developed as a rule-of-thumb based on how long it takes folks to stop adding fish to a tank, how long it takes them to understand maintenance, etc. There's nothing magic that happens after a year, but with how much is going on during that year it's safe to say water parameters are anything but stable. Our limited test kits for a few things may *say* it's stable, but there's a lot more going on then we can really test for.
Can you add an anemone after a month and have it survive? Maybe yes, maybe no. But I'm guessing enough folks have tried and failed, and the "collective wisdom" eventually got entrenched as a "rule".