I assume you had a filter going in the big tank and the water current may have been too much for your betta. Especially if he's not used to having a current going. Also, new situations, especially that involve new co-habitants, will be intimidating for most bettas for awhile--until they establish what the pecking order is and who is peaceful/who has to be avoided/who is aggressive. (Incidentally, I've heard that bettas and angels shouldn't be kept together--the fins bring out the worst in both fish and one will often kill the other.) Some bettas who have lived most of their lives in solo tanks may never adjust to communal living; however, some bettas come to like having other fish around. After you've determined he isn't sick, and if you still want to introduce him to a big tank, first put him in a smaller "tank" (or container--we have "specimen" tanks) while he gets used to seeing the other fish around him all the time. After a week or two (keeping the water in his little tank very clean--it'll get dirty faster than a big tank so do daily cleanings), release him but keep an eye on him. Make sure there are places he can hide in safety (if one of the established fish gets aggressive). After a while (might take a week or so), you should be able to tell if he's adjusting or if he's going to have to have a room of his own.