I was challenged on another forum to list the basics of koi keeping. So I wrote something quickly "out of my head" like I normally do. Here it is copied and pasted, with a few changes after I thought about the subject a few more days. Some of you may find the comments useful, or perhaps even controversial. Okay, either way, write what you think, let's discuss it, I am home sick with the flu, and bored stiff!
The first basic is do not plan to keep koi past their second year in a pond less than 1000 gallons. Koi will not develop a good body shape in ponds less than 1000 gallons when they are more than two years old, and it gets pretty hard to keep the water quality up in ponds smaller than 1000 gallons when your koi develop to their "normal" 24 to 30 inches in length size.
The second basic is put in enough filtration to keep ammonia and nitrite below 0.5 ppm, preferably lower, at whatever stocking density is chosen to be maintained. There is very little agreement on the "right way" to do filtration! There are many paths to a great filtration system, and most of them are not described in any koi hobby related books. Meaning there are thousands of ways to install a great filtration system, and millions of ways to mess it up!
The third basic is to keep some alkalinity in the water, meaning a pH over 7, preferably over 7.5, below 8.5, and chosen in a way where the pH can remain very stable. Without some alkalinity the biofiltration bacteria die, and when that happens, goodbye koi. How best to maintain alkalinity is very dependent on the variables of supply water alkalinity, stocking density, and water exchange rate. For those with high alkalinity in the water supply, nothing is usually needed to be added to the water, for those with little to no alkalinity in the supply water, the usual approach is to buffer the water with baking soda.
The fourth basic is to keep control of green water algae with either biofiltration or UV lights to avoid the pH bounce sunrise to sunset (low at sunrise, high at sunset), meaning the pH bounce sunrise/sunset should be kept below 0.3 pH units.
The fifth basic is to keep parasites under adequate control so they don't ruin the health of the koi, there are many different ways to approach that subject, no one true way has to be chosen for parasite control. But an effective way must be chosen. Otherwise, long term health of the koi are likely to be compromised. Parasites run a close second to inadequate filtration as the source of koi health problems.
The sixth basic is to prevent viruses (KHV and SVC are the most notable) from getting in the koi pond to give a wipeout of the hobbyist's koi collection. The ways to do that get pretty narrow, I must admit! But we don't all agree on the "right way", nevertheless.
The seventh basic is to feed the koi something good for them to eat, at the times they should be eating, without overfeeding them to foul the water. We certainly don't all agree on the details of this basic, but the koi do need food!
The eight basic is oxygen, without enough dissolved oxygen the koi will fail to prosper and eventually die. So some way must be chosen to keep oxygen level up, both for the koi and for the biofiltration bacteria. The ways to do this are many, however! So we don't all agree on the right way to accomplish this basic. Some of use shower and trickle tower filters to supply oxygen to the water and to the biofilter, meaning that is how I accomplish this goal. Some with submerged media filters run air stones in the filters, and I agree that is a very good idea, it just isn't the way I chose to design my filtration system. Some put air domes and air stones in the pond, that helps, but is not as effective as shower or trickle tower filtration.
The ninth basic is enough trace mineral content for good koi health and coloration. The biofiltration bacteria need the same trace minerals, so this basic is doubly important. In some ponds, the supply water has enough trace minerals, in others it is missing. The usual approaches to the water being lacking in trace minerals are adding bentonite clay (such as Koi Clay), Epsom salt to increase magnesium content, calcium chloride to increase calcium content. The symptoms of low trace mineral content is loss of koi coloration, low GH measurements, and high nitrites since nitrite conversion is more highly dependent on trace minerals than ammonia conversion.
The tenth basic is to avoid contamination of the pond by chlorine, soap, pesticides, and polluted rain from the sky. Or, if that is an issue, install on line activated carbon filtration to remove contaminants as they enter the pond.
The eleventh basic is to find some way to "refresh the water" to avoid pheromone and DOC buildup. Some prefer water exchange, others like ozone, using activated carbon to absorb the pheromones and DOC works. I have another preference in my peculiar situation, namely low level PP oxidation of the DOC and pheromones. As long as it is done well, with proper precautions, any approach can be made to work. If this is ignored, the koi won't grow well and their colors will be poor.
The twelth basic is to keep some injectible antibiotics on hand when a fish needs some help recovering from an illness! And the knowhow of how to safely use the injectibles.
And, hey, the thirteenth or first basic is to have some water for the koi, they do prefer to be in the water! The rules of thumb are to have 500 gallons of water per adult koi, which can be pushed down to 200 gallons per large koi with really superior filtration systems that are well maintained.