Most fish species were designed with their stomachs specifically in mind to consume foods they would encounter during their natural habitat, including living plants and prey that are rich in proteins, vitamins, and minerals. Bread doesn't provide as many of these essential ingredients in an easily digestible form for these aquatic animals - its gluten, yeast and carbohydrates may even lead to constipation or bloating!
One of the primary negative health effects that fish will encounter after consuming bread is extreme bloating in their stomach. This occurs due to large quantities of yeast (used to make bread rise) and gluten, both of which cannot be digested by them and therefore cause their stomachs to expand, potentially blocking off their large intestine and stopping them from passing feces properly.
Bread or crumbs should never be fed to freshwater and saltwater aquarium pets such as Koi fish, goldfish or any other freshwater or saltwater fish; this also applies when feeding fish in public places like park ponds and waterways - fish should only consume live or frozen plant and animal matter in nature rather than bread!