I could easily slam you with wiki entry on kashrut and jewish dietary laws but i’ll leave you with this one http://www.mechon-mamre.org/jewfaq/kashrut.htm within the jewish community there are many levels of observation
Catfish are not considered kosher because they lack scales. Kosher fish need to have scales and paired fins. That is why shark is not kosher, their type of scales are not considered scales.