1. You must have one bunch of anacharis (underwater grass) per square foot of water surface area. This serves as a natural filter and as food for the fish. It grows faster than the fish can eat it.
2. 50 - 60% of the surface of the water must be shaded with floating plants. Water lilies are great, as are water hyacinths, water lettuce or water poppies. Just be sure to compost them as they multiply and not allow them loose in a natural waterway.
3. You must have fish to complete the balanced ecosystem. I recommend common goldfish. Do not put Koi in your pond unless you have built a koi pond. They will eat all of your plants.
4. Do not feed your fish. They will become too big for the pond and upset the ecosystem. You will have an overpopulation problem and all of your fish will die.
5. Put in one linear foot of fish for each 25 square feet of pond surface area. If you have 100 square feet of pond, you may have 4 foot long fish, 8 six inch fish or 16 three inch fish and so on.
6. Do not allow turtles, crawfish, alligators, ducks, geese, dogs, raccoons, possums, muskrats, nutria or your children to swim in your pond.
7. Do not use chemicals!!! EVER! Add a dechlorinator when you first fill the pond and then when you add more than 10% water afterwards.
8. Do not worry about pH. It will take care of itself.
9. Remove any dead or decaying vegetation regularly so that ammonia does not build up and foul your pond.
10. Relax. Your pond will generally take care of itself. I recommend benevolent neglect as the best approach to pondkeeping.
For detailed pond information, see my articles at Backyard Ponds with the Pondlady