We don't have a "rule" but it is generally agreed that if you get something for free then you pass on the results for free also. When it comes to breeding from some fish that were given to you though (especially if they continue to breed regularly) I see no reason you can't sell some but also give some to someone else who would like them but is not in a position to buy them themselves.
I give away a lot of plants. All I ask, when I hand them over, is that the recipient continues to pass resulting cuttings on to others also in need.
It is now referred to, as smidey said, as "paying it forward" but this is just another way of saying "Do unto others as you would have them do unto you" (Matthew 7:12 I believe for the religiously minded)
On a similar subject though, nobody like a newbie who, in their first few posts, starts asking for free stuff!
Free stuff should be offered, not requested.