We are in the middle of constructing a natural pond(large) and a new Koi Pond.We filled them a few weeks ago and the water in the natural pond has gone brown and the water in the Koi Pond green. They are next door to each other!!!
Sally, even though your ponds are next door to each other you will find that maintenance for them will be different. I would look into filter systems with UV and water quality to solve your koi pond problems. You may be able to put plants in it if the koi are small. The pH should be checked and kept at a level of 6-7. One-half to two-thirds of the pond surface needs to be covered with plants if a filter system with UV is not being used. Beneficial bacteria should be used anywhere from once a week to once a month.
An aerator of some sort in addition to beneficial bacteria will help with your naural pond. There are dry bacterias available in bulk with application rates of 2-5lbs per acre. Natual ponds are easier to take care of than koi ponds. They can take sevral seasons to balance without using things like aerators and bacteria. Sometimes they never do. Take care planting your natural pond. Many aquatic plants are invasive and will take over a natural pond.
Hope this helps!