Goldfish have a keen sense of smell. Nostrils are located above their mouth and flaps direct the water though and out the back opening.
Goldfish have a unique digestive system among fish. They have no stomach therefore digestion moves rather quickly through the digestive tract, thus producing more waste than most fish. They even have teeth to start the digestive process.
After the teeth grind down the food it is pushed down the oesophagus where water is squeezed out. The oesophagus is where taste buds are and little hairs that keep the broken down food along its path. Then the food passes into an expandable section thought to be the stomach for many years but there are no enzymes or acids present here. It does hold excess food though.
But just before the expandable section is where enzymes and bile from the pancreas and gallbladder break down the food as it passes through. Proteins and fats are broken down here.
All along the tract enzymes are secreted to break down carbohydrates and covert to sugar. Mucus is produced from this expandable section right to the anus. This is where the nutrients are absorbed for energy, growth and repair.
This process takes about 16 hours depending upon temperature.
Goldfish should be fed several smaller meals rather than one big one as the latter passes through with much undigested food, causing more waste. And more waste equals deadly ammonia that is toxic to the fish. However it should be noted that properly cycled tanks convert ammonia to nitrates which are non-toxic except in high levels.
Goldfish are omnivores and should be fed a varied diet to ensure proper nutrition. A dry food should be chosen as a staple preferably one with pro-biotic. Avoid flakes as the fish will take in excess air while nibbling from the top and produce gas. Sinking granules tend to be richer in nutrients and are known to cause less problems. They should be pre-soaked to minimize any air in the food. Feeding flakes twice a day is not the way to treat goldfish and their needs.
Some foods to include can be blood worms, brine shrimp for protein and also fruits and vegetables for the nutrients. Ensure any hard vegetables are cooked or finely grated and any hard fruits are finely chopped. One thing about fruits and vegetables is that they should not be a main part of the goldfish diet, they should supplement a staple food. Blanched un shelled peas are one way fed for several days to clear the digestive tract. Live plants can be a source of food and give them something to nibble on throughout the day.
Worms and such from the ground should not be fed as they can contain parasites and pesticides. Avoid breads as they can ferment in the digestive tract and causing gas which will affect the swim bladder.
Food can be the cause of many problems with goldfish including bouncy and swimming troubles. They will eat as much as possible causing these problems is why they should be fed minimally and more often than as one big meal. This type of feeding mimics them in the wild where they would be constantly grazing. Fasting one day a week is also recommended to clear their tract.
A word about protein: 60-80% protein should be fed for growing goldfish less than one inch. 40-60% for young fish and females which are producing eggs. 30-40% for older fish and maintained diets.
Feed your goldfish accordingly to provide a long and healthy life. They will thank you and be less susceptible to disease and swimming problems. Enjoy goldfish keeping!