I used to live in Israel and have recently migrated to Sweden. I am a Jewish atheist catfish enthusiast.
I am quite ignorant on Jewish religious laws, so you will have to forgive me! Maybe someone who actually is religious could comment and explain about how it works for them.
Indeed, religious Jews are not allowed to eat catfish. As for the various armoured catfish - I have no idea.
As for keeping them - I am not sure! I suppose it would depend on how religious one is. Dogs are unkosher animals, for example, but you would still find them in many Jewish homes!
On the other hand, I saw a TV programme with some very religious Jews who, at the time, were buying some things for their new baby. They avoided things that had childish drawings of horses on them, because horses are unclean! I have never previously heard of someone being so strict, however.
Once, in a pet store in Israel, I saw a religious Jew coming in with his child to buy some fish. He also bought some fish food. This was during passover, which has special dietary restrictions- you are not allowed to eat leavened bread and as an extension many other things (pretzels etc.). He asked the sales person if the fish food was kosher for passover. I don't think the salesperson knew! Anyway, dog food IS available in kosher for passover versions around passover - despite the fact that dogs themselves are unkosher. It is not as illogical as it sounds as the religious laws say that on passover you are not allowed to have such food in your home.
Okay, now I have been trying to google for more information on uncleanliness and how it works. My thinking was that maybe after you touch an "unclean" carcass and become "unclean" that perhaps you can... clean yourself? Somehow? Atone? In the little time I put into it I did not find anything specific about fish other than how to tell if a fish is kosher or not...
AHA! Now I found something!
Here is a link to someone asking about the kosherness of keeping unclean ornamental fish:
http://www.halachayomit.co.il/DisplayRe ... readID=349
I will translate the question and the parts of the answer that are the most relevant. I would rather not use the "quote" code and all as it is going to be a translation, and possibly a bad one at that ;).
"Question: Is it allowed to keep at home unclean animals, such as unclean ornamental fish or hares (Sfamnun: I assume they mean rabbits) and parrots and so on, or is it not allowed according to the halacha?"
Well, the gist of it was that it is allowed to sell unclean animals (such as horses or donkeys) as long as they are not meant to be eaten.
Speaking of donkeys, the messiah himself is supposed to ride on one, as Jesus has done! That would show to me that animals which are unclean for eating do have their place even around people who are supposed to be the cleanest possible.
"And so it is for fish, that the most of which are intended for eating, it is not allowed to trade them, and therefore it is forbidden to keep such fish at home even if you are keeping them for ornamental purposes. But really, most of the kinds of fish sold for ornamental purposes are not habitually eaten and therefore it would appear that it is allowed to keep them. And it was also written by <some people, I don't know who.> about birds who speak (speaking parrot) and other ornamental birds, that are not at all habitually eaten, that it is allowed to trade them and raise them and there is no prohibition on this at all."
As an example of an unkosher fish that is used more for eating than for ornamental purposes they mentioned the eel.
Furthermore they are saying that anyone keeping animals at home should beware to not cause them any agony/sadness.
It is also mentioned that furs are allowed to be traded and worn- even if they are from unclean animals. I have always wondered about this as the most religious Jews sometimes have big fur hats made from unclean animals.
Well, this post ended up being much longer than I thought possible!
I still haven't found an answer to how proper it is to touch an unclean pet's carcass but perhaps it is allowed since you are not allowed to cause animals suffering and if you were to let a fish rot in the aquarium it would cause the others suffering? I suppose a net would be a good loophole though.
As for how logical such laws are - I don't know how logical they were even back in the day they were written. Much of it I am sure is not just about health but also for social reasons. My favourite little bit of logic though is the kosherness of the locusts - the only kosher insects! My personal idea of how it came about is as follows:
Farmer: I'm hungry.
Leader: Eat some grain from your field, then!
Farmer: I can't, it has all been replaced by these delicious and yet illegal sky shrimps.
Leader: Hmmmm..!