I think I mentioned this, and not sure if it makes any difference, but he doesn't react badly until about 48 hours after he's been in the larger tank... he eats fine, swims fine, goes to his favorite hiding place, etc. Acts completely normal but then after awhile, he goes into shock. (I read about shock and thought it sounded like nitrite shock (or nitrate, I can't remember now). But, through several water changes and filter cartridge replacements, I got the nitrite and nitrate levels are the same, and the second time I put him in there after that, he still went into shock. And, it took him about ten minutes to "recover" once he got back into the small tank.

He's such a brat.