There are also facilities in Mexico caring for endangered and extinct Mexican fishes. You may have noticed that the Xiphophorus center will sell excess stock as a fund raiser (they also are looking for charitable contributions to an endowment fund). The Mexican fishes that are in trouble may have endangered or at risk status in the country, but they are not on the cities list (of the animals and animal products prohibited in international trade by treaty), so my GUESS is that if the authorities at one of those facilities had extra youngsters of a stock, they could issue papers allowing those specimens to be taken out of the country. So long as that is up front and payment/ a contribution is recorded in the accounts of that group, I wouldn't think that that would be illegal. And the facility might receive some very needed funds in exchange for fry they didn't have room for anyway.

They wouldn't do it for me if as a tourist, I just showed up and made inquiries. However if the person was a well known researcher or collector who contacted them ahead of time and negotiate the exchange, the response might be different.