It's a very young tank that's moving very fast. That's usually enough.
Anemones (even the "easy" ones, which a sebae is not) require pristine, stable water conditions. Even if your tests are running zeros across the board there are most likely trace amounts of nitrates and/or phosphates in your system that are not showing up because they're being used up before they show up on the tests. There is also something to be said for waiting until you "know" your system before you add the more difficult animals.