Water conditions are the most important aspect of Koi keeping and should be mastered before introducing expensive Koi or too many Koi.
If your water conditions are not good, your Koi will not grow as fast, they will get sick regularly or they will die.
In short, the waste from the fish creates Ammonia which is deadly to the fish.
 However, nature provided a nitrification cycle to solve this problem. Bacteria called Nitrosomonas grow in your Bio filter and pond and breaks down the ammonia into Nitrites. Nitrites are also deadly to fish, but Nitrites are being broken down by another bacterium called Nitrobacter into Nitrates which is beneficial to fish and plant life(but should also not exceed 50ppm).
The major reason for Koi deaths is overcrowding. A pond should NEVER have more than 10cm of Koi per 150 liters of water. I recommend at least 200 liters of water for every 10 cm of Koi.
Everything must be in balance like with everything in life.
To little Nitrosomonas causes too much ammonia, To little Nitrobacter causes too much Nitrites. The amount of fish, water volume and size of your biofilter MUST be in balance.
The biggest problem, a new pond and new biofilter needs time to create these bacterium. (at least two months) Introduce to many fish before the pond and filter becomes mature, will cause an imbalance. A pond takes up to one year to mature properly. Medicate the pond with certain medication without bypassing your bio filter will also cause an imbalance because the medication like Potassium Permanganate will also kill the Nitrosomonas and Nitrobacter. Cleaning your Biofilter to often or with normal tap water with chlorine contents, will also cause a problem. 
Another problem, these two very important bacterium does not really grow in the winter. When the water temperature heats up in summer, the fish eat more, produces more ammonia, but the beneficial bacterium has not grown yet to sort out the Ammonia and Nitrites. This explains that most major outbreak of infections and poisoning in ponds happens in spring.
Hobbyists also tend to clean their ponds less frequently during winter. Leaves, sludge and other debris builds up, creating "bad" bacteria. As soon as the temperature increases, outbreaks follow.
Therefore, it is a good idea to ensure that your salt contents (which will kills most harmful bacteria/parasites) at the beginning of summer remains at least 0.3% and limit food intake during the first month of spring.
The chemical balance should be tested on a weekly basis, and every time any changes in the pond takes place.
At the beginning of summer, more regular testing should be done.
Therefore, a proper pond test kit is a must and is available at most Pet Shops.
Minimum allowed :

Ammonia
less than 0.1 ppm(depending on temperature)
Nitrites
less than 0.2 ppm
Nitrates
less than 50 ppm
pH
6.7 -8.5 
Alkalinity
60 - 170 ppm
Hardness
75 - 150 ppm
Chlorine
less than .04 ppm
Oxygen
more than 5 ppm