FEEDING
Feeding Koi
Always feed your koi in the same place.  In this way, they will become very tame and learn to eat from your hand.  By bringing the fish in close, you can monitor them for any loss of condition or disease.  You will also notice if any of them are missing or sick.  It is better to underfeed rather than overfeed.  Because koi do not have an organ equivalent to a stomach, they cannot eat a great quantity of food at one time.  So therefore feed little in one setting.  Feeding your fish too much in a one setting might give your filter a hard time to clean your water due to excessive waste.  I usually feed my fish twice a day with variety of food.  I feed them thrice a day only during weekend so I can monitor water quality.
Pellets
Pellets come in many size and many kinds it got small for smaller fish, medium and large.  We've got cheap ones which is poor in nutrients of course.  Some concentrating in color enhancer, some for growth so read your label correctly.
Flakes
Flakes also come in many kinds just like the pellets but not ideal food for big koi cause it is too thin to satisfy them that means you have to feed them more in flakes.
Sticks
Koi sticks one of the favorite food of my big koi also got many kinds just read your label.
Earthworms
Can be found in your garden or you can go to your nearest petshop and bought some or just go to your backyard and find one.
Daphnia
Waterfleas can be bought in pet stores or just collect them in your pond especially during hot weather.  But just make sure that it is a reliable source so you can make sure it is a disease free.
Tubifex Worms
Can also be obtain in pet stores just like Daphnia make sure it is safe.  But i usually bought the dried one which is certain it is safe.
Bloodworms
Can also be bought in pet shops also be sure if it is safe.  I bought the dried and frozen ones.
Mosquito Larvae
Very easy to have your own quality safe source just put a basin in the backyard and fill some water after couple of days you've got your own good source of mosquito larvae.
Glassworms, Maggots, Water lice, Tadpoles, Plants
Which you call all obtain these in your pet stores.
Small Fishes
These are also a good source of food for koi.
Brine Shrimp
You can avail it in your local pet stores
Here It is being processed to flake which also can be bought in pet stores.
Dried Shrimps
One of the favorite of my koi can be obtain in pet stores too.
Forms of Food
Home
Stick-on tablets
Can be bought in your local pet shop ideal also for koi cause it eat in all  layer of the pond. Ideal for middle eating fish.
Sinking pellets
Ideal for bottom feeders.  Can be bought in pet stores.
Sinker (sinking granular food)
This type of food sinks very fast and very good for bottom dwellers. Can also be bought in pet stores.
Drosophila
The fruitfly don't know if it is available in pet stores but it is a good food for koi.
Wheatgerm
It is nutricious food can be bought in some pet stores.
Brown Bread
A good source of carbohydrates. Avoid white bread.
Boiled potatoes
A good source of carbohydrates
Peas and beans
Nutricious food.  Use sparingly
Sweetcorn
Also a good source of carbohydrates
Lettuce or spinach
Feed chopped or shredded lettuce occasionally
Processed Foods
Live Foods
Ohter source of food