Sounds like you will feel really guilty if you steal them. Personally, however little they feel for their fish, I imagine that having something stolen by someone who is a familly freind and trusted to some extent would be a terrible feeling.
Also, the chances are that all that'll happen is they'll realise its you and replace the fish -it's not really getting you any further forward.
Ask them directly if you can take the fish from them, as you like them and they've said they find them boring. If you have to, offer them some small amount of cash for them. I'd go for raving about how much you like their cory, which will either mean they'll value it more themselves or else be more prepared to sell/give to you.
Also, the chances are that all that'll happen is they'll realise its you and replace the fish -it's not really getting you any further forward.
Ask them directly if you can take the fish from them, as you like them and they've said they find them boring. If you have to, offer them some small amount of cash for them. I'd go for raving about how much you like their cory, which will either mean they'll value it more themselves or else be more prepared to sell/give to you.