Not performing the necessary due diligence in regards to fish selection, i.e. adding THIS guy
to a heavily aquascaped 500g aquarium. It lived for six years and killed about a half dozen fairy wrasse during its 'tenure'.
Also, my tendencies to be both impatient and stubborn have led to several of my greatest mistakes, such as continuing to add the aforementioned fairy wrasse to the scenario above instead of just tearing down the tank and removing the pseudo. But I'm getting better about it all now. This is due to my becoming a major proponent of the 'If it ain't broke...' theory of reef keeping. Of course, there
was the Crosshatch incident last year, and the urge I've been having lately to add just one more fish to the tank...ok, maybe two. But other than those few things...