[MMM]

Probably because many Japanese alive today still remember the WWII there is not a lot of patience for picky eaters or wasteful eaters because there was a time in recent memory when people didn't have food.

Tell what ever program you are going on NOW that you need to be placed in a home where not eating fish is an option. This way the family is not surprised when you arrive. I would say you are allergic to all fish, just to make it easier for you. There may be host families that have a child who is also allergic and it will be a perfect match.

That being said, not eating fish in Japan is not that hard to do. The important thing is that your potential host family know this in advance.