What kind of fish, what kind of tank etc?
I'm laughing because I watched my goby leave a poo behind the other day - was the first time in several years of saltwater fish keeping that I've actually seen a fish do that :p
Generally speaking your filtration should remove any negative effects that the poo would have - and regular water changes will get anything leftover :p
firefish, ocellaris clown fish
caulastrea, green star polyps, palythoa, zoas