Are there any kashrut laws regarding fish? I do not mean which fish to be eaten because this is spelled in Torah (Lev 11) but rather how, when. Is there anything that makes a "clean" fish unfit to eat? Are there any laws regarding killing the fish, cleaning of blood etc. (like similar laws regarding animal meat). Is it ok to buy fish from any market/store? I know a friend that does not eat fish unless he catches it and kills right the way it or he buys it alive. If there are any such laws can you also reference where are they written. I would appreciate an answer regarding this subject.

There are no laws regarding the eating of kosher fish other than to be certain they are kosher, that they aren’t wormy, and that they don’t have bits of non-kosher fish attached to them (if purchased in a store that sells non-kosher fish as well). It is customary to eat fish on Shabbat, and one reason mentioned (hassidim) is so that they be connected to a Mitzva.