Whoa, I almost got handed a giant just like him the other week, but that thing was huge with a nasty temperament.
Anyway, you did the right thing, even if you weren't prepared for another fish - you've saved a life and I'm sure that if you decide to keep him he'll repay you with years of fishy dancing and bubble nests. The setup you have him in isn't the end of the world, it's fine as a temporary home and certainly better than being flushed - which I personally find to be an absurd concept, killing an animal just because you don't want it. Unless people think that they go into the ocean or something, which is even more ridiculous.
Keep us updated on his progress :)