Stringy algae can occur even in ponds with clear water. It usually grows on waterfalls or close to the surface, and can only be removed by pulling it off by hand. A good rule of thumb is one inch of fish for each ten gallons of water. Also, it is important that you properly calculate the gallons in the pond. Multiply the average width by the average length by the average depth, then multiply that by 7.5.
Most people think they have way more gallons than they do.
Feed goldfish once every two or three days. Feed them all they can consume over a five to ten minute period. If you are finding food left beyond that, then you've fed too much, and you should reduce the amount next time. Koi can be fed a lot more, especially if you are trying to keep them from eating your plants. Once or twice a day is good for them if you have a good filtration system and a correct stockage of fish for the size of the pond.