I know of people who feed their pond fish catfish food and even dog food. They smile and tell me how cheap these foods are and that their fish do just fine with them. This may be true — for a while. Eventually, however, the "cheap" ingredients in these foods will cause the fish to become obese and die.
These types of foods are not formulated for ornamental fish. They contain grains and other plant proteins, not the main diet of any fish in the wild. Have you ever seen a fish eat corn on the cob? It is true that catfish food will "fatten ‘em up" quickly. That’s what it is designed to do — fatten the fish up for the kill — as quickly as possible so that they can be caught, cleaned and cooked. I doubt if you would want to do this with your koi and goldfish?
Ingredients to Forego
I am assuming you want to keep your koi and goldfish healthy and living for as many years as possible, as opposed to eating them. For that you need food especially developed for the long-term health and growth of the fish. There are many foods on the market that are designed for pond fish. Some are better than others. Some contain the "cheap" filler ingredients that add to the fatty growth that can play havoc on the fish’s liver later in life. Our best advice is to take the time to read the ingredients listed on the label of the food. Avoid the ones that have ground corn listed as any one of the first 4 ingredients. (There will be a higher content of the ingredient in the food if it is listed at the top) Many grains are a cheap source of protein and tend to be hard for ornamental fish to digest. Most of this protein goes through their system without being digested and out as waste, taxing your filtration system. This adds more ammonia to the water. If you are having problems with ammonia build-up in the water, check your food for a high content of grains.
Ingredients to Accept
A good food for your pond fish should list fish or other seafood meal as the first ingredient unless it is a wheatgerm food. If wheatgerm or wheat flour is listed first it should be followed by fish meal. Definitely avoid foods that list ground corn as one its first 4 ingredients. Corn Gluten, however, is acceptable as the 4th (or later) ingredient. Corn Gluten is the most easily digested part of corn.
The food should also contain Vitamin C, the best form of which is stabilized ascorbic acid (ascorbyl-2-phosphate). Vitamin C is very important when helping strengthen your fish’s immunity to bacterial infection. Other supplemental forms of high Vitamin C can be achieved by feeding sliced grapefruit, oranges, watermelon and dark lettuces (not iceberg).
Some foods have a color enhancer added (normally spirulina, same as "seaweed"). Spirulina aids in the brightening of some of the colors in your fish, especially the reds. Koi enthusiasts often feed their koi a high content of color enhancers just before the fish is judged in a koi competition to bring out the best coloration possible. Too much (or a continuous feeding) of the color enhancers can turn the whites of the fish pink. 2% spirulina added to the food is just about perfect for long-term feeding.
How Much is Too Much?
How much you feed your fish depends mainly on how well your bio-filtration can handle it. You can feed your fish several times a day (right, Gaye?) during warmer weather but only if you have enough filtration. Remember, the more you feed them, the more they grow, the more waste they give off, the harder your filtration must perform. For an average pond when water temperatures are between 65° and 70°, a rule of thumb is to feed twice a day and never more than they can eat in 5 minutes. After the water temperature reaches 70° you may feed more often as long as ammonia does not build up in the pond. Always remove any uneaten food.
Storing the Food
Care should be taken when you store your food. Large quantities can be kept fresh in the fridge (not the freezer). Never leave the food exposed to the sun. It will become rancid in only a few days.
Most of us who have fish in our ponds become attached to our finny friends. We name them and love them just as we do our cats and dogs. A koi that has been cared for well averages about 50 years of life. A goldfish about 15 years. They are part of our family. When we lose one we cry. I want my pets to have every chance to survive and to feed them inferior food would lower those chances.