Its sad about your clown. I bet, that your clown had one of the internal diseases. It does not relate to water parameters or type of fish, it just happens. I does not have experience with that myself, bud heard from my friends. The fish is happy, eating, swimming well in the morning and is dead without any visible signs in the evening. Just like she is out of any life energy. I think there is no prevention against this. My friend's tank is about 4 years old and till now she had no dead fish. And now, without any reason, any water quality or food change, three of her fishes died - chromis, mandarin and pipe fish. Sad story.