Transparant goldfish are useful for scientific purposes, offering a realistic alternative for dissections.

Once food is in the goldfish’s mouth, it is pushed to the back of the throat where a set of teeth grind and crush it against a hard pad.  There is an excellent photograph of goldfish teeth here.

The ground down food then passes down a tube called the oesophagus (pronounced oss-SOF-a-gus), which squeezes out excess water.  The oesophagus is lined with taste buds, little muscular hairs that brush the food along, and cells that produce mucus to keep things moving on.

The oesophagus empties into an expandable section of the goldfish’s digestive tract that for many years was thought to be a stomach.  However it does not produce any acid or enzymes to distinguish it from any other part of the gut.  It is simply a buffer zone to hold excess food as required.

Just before this expanded section, chemicals from the gall bladder and pancreas are pumped in with the food.  The ones from the gall bladder make up bile, which is used to break down fats; the ones from the pancreas contain enzymes that are used to break down proteins.

All along the digestive tract are cells that secrete enzymes that act on carbohydrates, breaking them down into sugars.

From the expanded section to the goldfish’s anus, lots of mucus is produced and as much useful material as possible is absorbed into the bloodstream to be used for energy, growth, protection and repair.

The whole process takes about 16 hours at an ideal temperature, but will slow to a stop in ponds during cold periods.

This is the reason goldfish should not be fed when it is cold.  Any food they do ingest can end up rotting inside them.  Any they don’t ingest can end up rotting in the pond water.

The nature of the goldfish’s digestive system also illustrates why it is vital to feed small meals throughout the day, rather than one or two big meals.

The food eaten in big meals will not be in the goldfish long enough to be broken down properly.  Most of it will come out the back end before digestion has a chance to act on it.

Related article:

Goldfish diet and nutrition

Forebrain

At the front are the smell detectors, technically known as the olfactory lobes.  They are directly connected to the fish’s nostrils by the olfactory nerves.  The information processed in the olfactory lobes is passed to the telencephalon, which “understand” what the smells are.

Together the olfactory bulbs and the telencephalon are known as the forebrain.

Research has also indicated that the forebrain may play a part in learning and retaining automatic responses to things that are unpleasant, and also in spatial awareness.  One experiment found that goldfish with damaged forebrains were less successful at memorizing a maze.

Finally, some fish make noises, and this is also thought to be controlled by the forebrain.

Midbrain

The midbrain (aka mesencephalon) sits directly behind, and is connected to the forebrain.  It’s main purpose is to process and understand the signals sent from the fish’s eyes to the brain.  It is responsible for sight.  In fish that hunt by sight, this part of the brain is comparatively larger.

Other functions of the midbrain include learning and controlling muscular reactions.

Fish are also thought to be able to tell what direction a sound comes from by using the midbrain.  (Fish have bones in the skull that form an inner ear).

Hindbrain

The hindbrain (aka metencephalon) is mostly responsible for controlling swimming, including the actions of the swim bladder.  It decides how much work a muscle does, how fast it works, and works out the balance of the fish.  Overall, the hindbrain controls what direction the fish swims.

Although the hindbrain is the largest part of the brain in most fish, those that swim fast and hard have the largest of all.  Fish that spend most of their lives latched on to rocks or other fish don’t have a large hindbrain.

Another function of the hindbrain is to detect chemicals that have an effect on blood sugar levels.

Other

There are several other smaller structures in the fish brain that control functions relating to hormones, breathing, salt intake, signals from the lateral line and reacting to all the other organs in the fish, and stimuli it will encounter.

The lateral line runs down each side of most fish (and amphibian larvae), from the gills to the tail.  It is usually visible as a faint line of dots about halfway down the sides of the fish.  The exact appearance of the lateral line varies from species to species, and these differences are sometimes used to help distinguish closely related fish species.

On a smaller scale, each dot on the line is called a neuromast.  A neuromast consists of a bunch of hairs surrounded by a gel cap.  This whole structure sits in a little hole in the side of the fish, and is connected to a nerve.

The general purpose of the lateral line is to detect changes in water pressure.  The gel cap transfers the pressure changes to the hairs, and on to the nerve, working much like the hairs of the human inner ear.

For example, when a fish is swimming normally with clear water ahead, the pressure is consistent and as expected, a bit like the wave that forms in front of a boat, only in 3 dimensions.  So when the fish is heading for the glass of an aquarium, the pressure of the water flowing over the head and body will change as the shape of the wave changes.  This enables the fish to avoid such invisible objects.

Changes in water pressure will also be caused by other fish swimming nearby, and the lateral line has also been shown to play a part in schooling behaviour in some fish species.

Other species can detect and accurately locate insects on the surface of the water, without having to rely on sight.  This is a good thing in an environment where light does not behave as predictably as in air.  It also means the fish does not need specially adapted eyes that point upwards, and can use them as normal i.e. seeing morsels in close view in front of the fish, and looking out for danger to the sides.

Low frequency sounds are also detected with the lateral line, simply because they cause changes in the water pressure.

Finally, in sharks the lateral line is used to detect changes in electromagnetic fields in water.  A very useful tool for hunting prey.

The reasons for goldfish losing their appetite fall into two main categories: goldfish diseases and environmental stress.

Goldfish Diseases

Constipation causes loss of appetite in goldfish. As regular readers of this blog will know from Goldfish Diet and Nutrition, goldfish do not have a stomach for holding and digesting food. Instead food passes through a goldfish’s digestive tract without stopping. If there is a blockage at one end there is no buffer zone to absorb the impact of food coming in at the other end, so feeding will stop.

A goldfish suffering from goldfish ick can also lose its appetite. Although not a guaranteed symptom of goldfish ick, it can occur in more advance cases of the illness and is due to a general and deeply unwell feeling caused by a goldfish disease that can eventually be fatal.

There is one more disease that can cause a loss of appetite in goldfish, but it is very rare among them. It is known as Hole in the Head Disease, or Freshwater Head and Lateral Line Erosion. More common among other types of freshwater fish it is a dangerous, but treatable condition that can be a cause of loss of appetite.

Environmental Stress

A change in diet can cause goldfish to go off their food. Obviously this is one of the easiest causes of appetite loss among goldfish to identify. If goldfish are changed onto a healthy diet from a poor one, they may well not eat for a few days. Their reticence to eat is due to being used to the old diet, and the slight stress of the change that may worry them until they get used to the new food.

Another environmental factor that may cause goldfish to lose their appetite is temperature variations. Again, regular readers of this blog will know from Preparing a Goldfish Pond for Winter that the natural seasonal drop in temperature can result in goldfish going the whole Winter without food. In parts of the world where Summer temperatures can fall sharply, pond goldfish can lose their appetite as a result.

Changes in water quality can also cause goldfish to stop eating, especially pH levels outside the optimum range. Goldfish that are put into new tanks that have abnormally high levels of ammonia and nitrite will also be susceptible to appetite loss.

Introducing goldfish into a new environment e.g. a new aquarium or pond, or bringing them back from the pet shop may well lose their appetite simply because of the stress of the move.

Finally, goldfish that see a new face pressed against the aquarium glass at feeding time, or goldfish that have to suffer a child splashing the surface of the water or tapping the aquarium glass will be frightened out of feeding. The same goes for pond goldfish that fear predation by a cat or heron. They simply will not surface for food, and will not emerge from hiding.

The Names of the Fins

a. Pectoral

b. Pelvic

c. Anal

d. Caudal

e. Dorsal

Stability

Goldfish move in 3 dimensions, therefore have 3 directions they need to maintain stability in.

Firstly, imagine a model goldfish with a rod running through it from the mouth to the tail, with the model spinning around the rod. Goldfish use their anal, caudal and dorsal fins to prevent this movement.

Secondly, imagine the rod is running through the model from top to bottom i.e. from the dorsal fin to between the pelvic fins. The movement on this axis is also controlled using the anal, caudal and dorsal fins.

Finally, imagine the rod is going through the model from side to side, producing movement that makes the head and tail go up and down. Goldfish use their pectoral and pelvic fins to control this movement.

Direction & Steering

Goldfish move their caudal fin from side to side for forward propulsion. The pectoral fins are used for more delicate movement, both forwards and backwards.

Goldfish steer with their pectoral fins, and also use the dorsal fin when turning sharply.

In the wild, goldfish feed on algae, plants, plant roots, worms, insects, larvae, tadpoles, small fish, snails and anything else they can fit in their mouths to taste and check if it’s ok to eat.

Although aquarium and pond goldfish are not wild and look nothing like wild goldfish, they have exactly the same digestive tract, and need the same sort of diet.

Feeding them flakes from a tin twice a day is not the way to do it.

Mealtimes

As stated above, it is best to try to mimic what goldfish would experience in the wild. As they would be almost constantly grazing, small meals are better than large meals. If goldfish are given a lot of food one or two times a day, they will pass most of it undigested. Goldfish do not have a stomach to hold food in while it is broken down, it just keeps on moving.

Dried Flakes, Pellets and Tablets

These types of goldfish food should be used as a base in their diet, but not as the only source of nutrition. The better ones will provide fibre, vitamins and minerals – as well as some quality protein, but they will not provide a varied diet.

Also, beware of flakes that are bulked up with cheap corn starch, or use cheaper vegetable proteins instead of fish proteins.

Another point worth noting is that dried foods swell up when they get wet. If they swell inside a goldfish, they may cause the swim bladder to be pressed out of shape, causing problems for the goldfish.

Frozen and Freeze Dried Food

These types of food are high in nutritional value, and there is plenty of variety. They are high in protein and contain all the amino acids that goldfish need for healthy muscles and tissue.

Freeze dried foods also need to be pre-soaked because, like dried food, they swell up when they get wet.

Live Food

Live food such as daphnia, freshwater shrimp and midge larvae is the most nutritional of all for goldfish. However there are a number of problems associated with giving live food to goldfish.

Firstly it is much more likely than any other goldfish food to carry disease or cause illness.

If worms or larvae are harvested from the wild, it is vital to ensure they do not come from ground that has been exposed to pesticide.

Secondly, they are harder to obtain than other types of goldfish food, and it is often necessary to breed them in separate tanks.

Finally some species of worm can disappear into gravel and set up home in an aquarium, causing problems for the goldfish and the aquarium.

Fruit and Vegetables

Certain aquatic plants can be grown in separate tanks and fed to goldfish, and they also enjoy a wide range of fruit and vegetables.

Make sure hard vegetables like carrots are cooked or finely grated, citrus fruit sections have the skin removed, things like peas and grapes are peeled, and softer items like lettuce and apple are chopped up. Fruit that is made of small constituent parts like raspberries can be dropped in whole, and the goldfish will pull the smaller manageable parts off.

One word of warning about fruit and vegetable – use them as treats and nibbles, not as a major part of a goldfish diet.

Firstly, make sure the goldfish are mature enough to breed. Any goldfish under 8cm (4″) is probably not ready for breeding.

Then, make sure the goldfish are getting as good a diet as possible. Feed them a broad range of dry and live food.

Next, make sure the water is at a stable temperature of about 18 Celsius (65 Fahrenheit). The reason for this is to bring the water temperature up by a couple of degrees Celsius when the goldfish are ready to spawn. This mimics the natural conditions when winter turns to spring.

Obviously this is easier to control in aquariums than in ponds, however the rise in temperature is more important than the actual temperature, and that should occur naturally in ponds.

Finally, make sure the goldfish that are entering the breeding tank are not the same sex. This previous blog has information on how to tell the sex of goldfish. Better results are achieved by putting 3 males in with 2 females.

The Spawning

When the goldfish are ready to spawn, the male will have the breeding tubercles on his gills. Raise the water temperature by a couple of degrees to 20 – 22 Celsius (68 – 72 Fahrenheit).

When spawning takes place, the female goldfish will lay hundreds, possibly thousands of eggs in the water. They are sticky and need something to stick to. Spawning mops or plants need to be in the water for the eggs to adhere to.

After the female lays her eggs, the male goldfish releases milt (sperm) to fertilize the eggs, and the water goes cloudy.

About 3 or 4 hours later, everything will be over and the goldfish eggs should be removed from the aquarium. This is because the adults will eat the eggs, and if left in long enough, they will eat the newly hatched goldfish too.

Incubation and Hatching

Place the plants/spawning mops with the eggs into shallow water and check them a few hours later. Unfertilized eggs will turn white and need to be removed. They can develop fungus which will spread quickly if they are not removed. Fertilized eggs are a transparent orange-brown colour.

Depending on the water temperature, the goldfish eggs will hatch between 5 and 7 days later (warmer water for shorter incubation period). When the fry first emerge, they are still attached to and feeding from their yolk sacs.

They will feed from the yolk sac for 3 days, during which time it is very important not to feed them. Once the yolk sacs are finished, the young goldfish will be very hungry.

The best diet is a balance of live feed (daphnia and brine shrimp), powdered food (dried flakes and hard-boiled egg yolk) and liquid feed (available in pet shops), 3 times a day. As with all goldfish, take care not to overfeed and polute the water.

And that is the hard part of breeding goldfish over. After the young goldfish are established solid food eaters, all that remains is to nurture them. Don’t overcrowd an aquarium with them, don’t use strong pumps, and stick to good aquarium management techniques to become a successful goldfish breeder.

Breeding Signs

The best time to tell male goldfish from female goldfish is the breeding season i.e. Spring. This is when the male goldfish will develop small white pimples (sometimes called tubercles) on the outside of the gill flap, and along the front edge of their pectoral fins (these are the ones just below the gills).

Also at this time of year the female goldfish will get a bit fatter because she is full of goldfish eggs (properly called roe). However, please don’t assume that a heavier looking body means a goldfish can be called Mummy. It could be a symptom of illness or disease, so look out for other signs of trouble.

The Vent

The vent is the opening at the rear end of every goldfish that is used for defecating and reproduction. In female goldfish it is a round-ish shape and is slightly raised above the level of the skin. The male goldfish’s vent is usually more oval in shape and does not protrude as much.

Be aware however that like most methods of sexing goldfish, comparing vents cannot be done at a glance. A bit of experience and careful examination is required.

Midline Ridge

Male goldfish often have a ridge that runs along the back half of their underside ending at the vent. This feature is usually absent in female goldfish, but can be present and should only be used as confirmation of other observations.

Chasing

In the breeding season male goldfish chase female goldfish. Out of the breeding season male goldfish chase male goldfish. Therefore to use this method of sexing it is vital to have closely observed the goldfish for a period of time before making any decisions.

Other

With experience and careful observation it is possible to learn how to tell male and female goldfish apart by the thickness of their anal fins, the shape of their pectoral fins and the firmness of their abdomens. Some people have even claimed to be able to tell them apart by colour. However, these methods can only be used with any degree of confidence after years of experience of keeping goldfish.

Summary

Wait until goldfish are mature before attempting to find out what sex they are. It is very hard to tell outside the breeding season without a great deal of experience. However, taken together the signs detailed above can lead to a definitive answer.