The fish in the sewer can't be eaten no matter how long it is put in clean water. Fish that grow up in the sewer can only exclude some substances such as sand, bacteria and seaweed, but can't exclude parasites, chemicals and heavy metals, so they can't eat them. Fish growing up in the sewer have more chemicals, more parasites and higher hormone levels than ordinary fish. These toxic and harmful substances are all products of meat, so no matter how long they are put in clean water, these potential toxic and harmful substances in their bodies will not change much. Long-term storage in the sewer may also lead to some bad mutations or lesions in fish, and it is not stored in clear water. Therefore, in order to avoid physical injury, as long as the fish in the sewer are kept in clear water, don't eat them. There are many harmful substances in the gutter. Eating this kind of fish will do great harm to your health, so you'd better go to a regular supermarket or market to buy live fish.