Their oriniganl purpose was to monitor water pollution, the idea being that you could add the fish to a body of water, and then, instead of having to try and catch and count how many survived, you could just illuminate the water with UV light and see (roughly) how many fish were left.
Of course the company can't control the fish's breeding and I think they would probably be hard pushed to prosecute you if they just bred in the tank, but they are copyrighted, so you would be in trouble if you sold them for profit.
All genetically modified organisms are illegal in the UK, except under special licence for research; a lot of people don't agree with it and are worried about what effect it could have on the enviroment if the GM plants or animals escaped and bred with wild populations.