If a fish is commonly considered to have scales – even though its scales may have fallen off due to some incident – and it is caught alive in the water and dies out of the water, it is pure and lawful to eat. In the event that it dies in the water, it is pure but it is unlawful to eat even if it dies by means of something, such as poison; however, if it dies in a fishing net in the water, it is lawful to eat.
As for fish that are commonly considered not to have scales, they are unlawful to eat even if they are caught alive in the water and die out of the water.