Why do my fish get sick and die? Your koi pond (or any recirculating water environment) is a closed system; water stays there. Fish constantly feed and emit waste products in the form of ammonia nitrogen, or NH4. This makes the water progressively unfit for fish health.
The Nitrogen Cycle The biological event that cleans up the waste is called The Nitrogen Cycle. Beneficial bacteria in your pond constantly convert NH4 (ammonia, toxic) to NO2 (nitrite, toxic) and then to NO3 (nitrate, harmless), which is almost the same thing as plant fertilizer. Bacteria get into a new pond via water plants, fish, or using water from an existing pond. Over a couple of weeks, the populations of bacteria grow large enough to handle the waste produced by the fish. Bacteria do not grow very quickly. This is the reason pond water often becomes foul, even in an established pond, after a large addition of fish or a drastic increase in feeding.
Why do I get algae? Because your pond is basically a fertilizer factory, plants will grow very well in it. If you have few or no plants already, algae (singular, alga) will grow. Alga spores enter your pond by fish, plant, nets, and even air. Algae will not kill your fish, but most pond owners desire to get rid of it.
How can I prevent algae? If you have many plants in your pond, say 50% coverage or more, they will eat up the nitrate as it is produced. Algae have little chance to get started. Once algae gain a foothold, it is almost impossible to remove permanently.
What plants are good for algae prevention? Water Hyacinth are the quickest way to achieve the recommended 50% coverage in your pond. As they float the hundreds of feathery roots efficiently soak up the nitrate. They also provide surface area for your beneficial bacteria to thrive on. They will double in number every few days in good conditions, which are full sunlight, nitrate, and 75º-90ºF. You can thin them out or you may not be able to see your fish! Once your pond has a good balance of Water Hyacinth and fish, you can add water lilies and bog plants to your heart’s desire.
Caution! Larger (>8”) koi love to eat the roots, so devise a way to protect your plants! Options are
- Placing plants in the concrete gravel filter (you can even permanently remove all the gravel!) and waterfall area, and
- Section off an area of your pond that the fish cannot swim into.