Koi fish should only be given as much food as they can eat in a couple minutes, twice a day. Unless you have a large filtration capacity and you are trying to boost your Koi’s growth rates, giving them more that the recommended amount of Koi food will pollute the pond water.
Uneaten food and increased fish waste will lower the water quality. Poor water quality can lead to health problems with your Koi fish. Overfeeding can also cause your Koi fish to become overweight. Overfeeding is easy to prevent, just start giving your Koi an appropriate amount of food.
Overstocking Your Pond
It’s very exciting to have a pond swarming with several varieties of Koi. Overstocking can easily happen when the many small Koi that live in the pond turn into many large Koi. Filtration that worked just fine when the Koi were small might not be adequate enough to handle fish load when they are larger. Overstocking can be prevented with careful planning. Allow 150 to 500 gallons of water per fish in the pond. That GPK (Gallons Per Koi) allowance will provide them with enough room when they are mature. If your pond is currently overstocked, only reducing your stocking numbers or enlarging your pond and adding more filtration will solve the problem.
Not Enough Filtration
Under filtering a pond leads to poor water quality and poor water quality leads to poor Koi health. The filter system you have today might not be enough tomorrow. A pond might not necessarily start out with inadequate filtration, but as your koi grow, the amount of waste they produce will increase. Adding to and improving your filter system will keep your pond from from being under filtered.
No Quarantine Period
Adding a Koi to your pond without first quarantining it can introduce parasites, bacterial infections or viral infections into your pond. You should quarantine every Koi that is going to enter your pond. Even a Koi that you already own if it returning from a Koi show. Koi are usually quarantined in a 100 or 200 gallon horse trough tank with a filter. The quarantine period should last at least three weeks. If there are no problems by then, the quarantined Koi should be safe to introduce into your pond.
Leaving the Hose On
Forgetting that your hose is on after it is done refilling your pond can poison your Koi fish collection with chlorine (unless you have well water).
Koi pond chlorine neutralizer should be added before doing a water change. If the hose is left on for too long, the chlorine neutralizer will be depleted and the chlorine concentration in the pond will rise until it reaches lethal levels for your Koi. A hose timer can be used to keep it from running too long.