There will probably be some members who will disagree with me but if you can match tap and pond water temperatures and the tap and pond ph are close to the same, you can do a 100% water change if you want. Just be sure to dechlorinate the water if you are on city water. I would be interested to see your ponds levels of ammonia and nitrate. Are the fish's fins clamped? A UV sterlizer is not something I would recommend for you right now.The green water is helping to keep your water less toxic to the fish than it would be without the green water.