You gotta take it back.
Maybe this is a great opportunity to train your family to buy you gift certificates, or you could post a wish list on the side of your tank if they insist on bringing in fish. Equipment like nice filters, uv sterilizers or co2 unit if you want a planted tank or something, a certain decoration you've had your eye on, a really good aquarium book, things like this could be on your wish list along with fish. You have to train them not to bring in 'cool' fish, that that is not an appreciated gift. Better to start with bil than gf or mom, IMO.