Looks like you fish has a bacterial infection. Probably brought about in the start, by a parasite infection. The fish should be removed from the pond to a separate container and the wound treated right away. I have had wonderful success with fish that have cuts and infected abrasions, using Melafix. Follow the directions on the bottle as to dosage. I found that lifting the fish's body up, just above the water level in the isolation container,while wearing a cotton glove and pouring the prescribed dose of Melafix, directly onto the wound for the first couple of days, had wonderful results. The red soreness had dissipated from around the wound very quickly. In a week or two, any of the fish treated this manner, were back to normal, with the wounds completely healed. As I already mentioned, in a previous thread, on the same subject, I think you have to do something about the ducks and the condition of the pond water, or you are going to have continuous problems with the pond's water and more sick fish. "Horton"

I think I've realized that no matter how careful we are about our water, we don't always have the ability to really control the quality. From time to time there are all sorts of weird things that are coming through our tap that are safe to us, but not so safe for the fish and it hits them in some way resulting in illness or death. My brother lost all his fish one time when there was some surge of bacteria in the water. It just wiped out everything he had including 10 year old oscars. Just something to keep in mind when you know you've been doing everything right and suddenly something throws you for a loop! I've found mollies do best in a pretty high concentration of salt and do better in a tank strictly for brackish water fish (something LFS never really tell you!) I think you just need to remove your current stock of live plants, working on just ridding the algae, and then reintroduce the plants. But do realize that the algae will be also probably be resident ON your plants somehow - and ridding yourself of the algae in your tank doesn't mean you're rid of algae completely. I really love having live plants in a tank, but I've found they're just really a lot of work!! Okay - so I'm a lazy sort of fish keeper. I really like being able to just enjoy the fish swimming around happily without having to deal with planting medium, fertilizing, always having to keep trimming plants, sometimes having to thin things out so other plants get light, keeping algae out, AND keeping water parameters balanced. And then for true planted tanks, they always recommend additional CO2 for the best growth possible. Planted tanks are a lot of work. I used to subscribe to Aquarium Fish mag that has a regular planted tank column. The writer is really very informative. So while I love looking at a live planted tank with fish - I've been sticking to fake plants in my tanks! Only my pond fish outdoors get plants - and they always wind up eating those.

Let's start with a brief explanation of why you should not be feeding your fish during winter. As the water cools in the autumn, the fish start slowing down in preparation for a period of semi-dormancy. Their digestive and immune systems slow down, and they are not capable of digesting the same types of foods that they can during the warmer months. Koi are particularly susceptible to problems at this time, as they have no stomach to aid in digestion; food passes directly into their small intestines, where it will sit and rot. The ensuing intestinal bacteria can pass into their bloodstream, causing sepsis which will debilitate or even kill the immune-suppressed fish. The general rule of thumb is to slow feeding when the water starts to cool into the upper 50s farenheit, feeding small amounts of low protein food a few times a week, and to quit feeding altogether when the water reaches 50. (A thermometer can be purchased for minimal cost and should be suspended in the pond, about 18 inches deep.) If the fish are still hungry, they'll find algae and other natural delights to sustain them, and these simple foods found in the pond will be more easily digested. The above rule, however, applies only to those of us who live in a climate with cold winters. Those living in climates where the water hovers around the 50-55 degree mark have a challenge. As stated, the fish's immune systems are not working up to par at lower temperatures, however, many "bad" bacteria are still active at these temperatures and can create problems in the weakened fish. In these cases, a small quantity of medicated food fed once or twice a week can help the fish withstand the bacterial attack. (Some people also salt their pond at this time as a prevention against parasites that are still active at lower temperatures. There is good information on the Koivet website on salting the pond.) If you feed your fish during this time, be sure to monitor ammonia levels, as the fish will be giving off ammonia in their waste and the bacteria in your biofilter will not be working at these low temperatures. (When testing winter water, always warm the water sample to room temperature before testing.) As the water warms into the fifties in the spring, you can slowly start feeding again. Beware the false spring syndrome! Many people are excited at the prospect of the first warm spring-like days and the increase in fish activity, and begin feeding, only to find the temperatures plunging into the thirtys within days. *BE PATIENT* and wait until you're sure your water temperatures are stable before you begin feeding. Once the water reaches a stable 50 degrees Fahrenheit, it is a good idea to start feeding with a small amount (two or three times a week for no more than three weeks) of medicated food containing Romet or Terramycin, supplemented by low protein, high carbohydrate foods such as wheat germ, cooked squash, brown bread, dark greens (not iceberg lettuce), and fruits high in Vitamin C. As the temps head into the higher 50s, start adding a bit of low-protein pellets to their diet, and increase feedings to four or five times a week if the fish are hungry. As the water heads into the sixties, start increasing the protein in their food. By the time the water is 65-72 F the majority of their food should be a good quality, high-protein food. Avoid foods that contain corn and other plant proteins as primary ingredients; the protein should be from fish or seafood to ensure the fish are getting the proper balance of amino acids that they need to thrive. Fruits, vegetables, shrimp and other live foods can also be given during the warm, active months of the year. Observe your fish closely during spring warm up, as this is when they are most susceptible to bacterial and parasitic invaders. Do NOT overfeed, and be sure to scoop out any uneaten food so it doesn't foul the water. It is best to err on the side of under-feeding and allow your fish to eat the easily digested algae that is often prolific at this time of year, rather than over feed the wrong foods and cause additional stress to your friends as they make the often difficult spring-time transition. Ronaye, Steve and David

Koi will produce more waste, which will need to be either filtered or will break down in the tank and feed the algae. Do you really want the fish, or are you trying to solve the algae problem exclusively? Shading out the algae may be better, or using plants/moss to consume any nutrients in the water. Would cows drink from a tented tank? I haven't had my koi extremely long, but I really struggle to imagine them getting used to large thirsty cows on a regular basis. Could you possibly instead plant out water lillies in containers in the center of the pond, so the cows can drink around them, but the majority of the tank is shaded?