Yep...it happens to the best of us all though, sometimes. Did you just get something new, or are we talking about past offenses?
However, if you think about it, non-impulse buying is great because you know exactly what the fish needs, if it will fit in your tank, be compatible etc., and then you can go back and feel proud of yourself because you researched before you bought.
Just think about that when you feel the urge to impulse buy.