I kindly ask you as well to not take the information provided on here to heart.
You came on here asking for advice and our members have been kind enough to offer that. There are some points here which needed to be mentioned as possible causes for what is causing your fish to be behaving the way it is.
One of the things we as aquarists must take into account when we purchase a fish, is the type of environment it needs to survive and be healthy in. These if provided from knowledge gained not only from scientific studies but also from personal experiences, allow your fish to thrive and not be sick.
One of the major factors here is the size of your fish and it needs, the fish is showing you it is sick, it cannot tell you what is wrong...now we have to investigate what is causing this...this includes the size of it's tank, its feeding regime, water parameters etc.
It has been brought up that this fish is in a small tank which may be the primary cause it is getting ill...Goldfish
are known to produce a lot of waste and in a small environment and an inadequate filtration or cleaning system, this waste will quickly accumulate and turn the water toxic. It is exactly the same that would happen to use, if we didnt change the water in a bath tub and used it constantly...now this is an extreme example but put it into context for your fish.
It has already been removed from an environment stressful to it, in a crowded tank in the pet store, a tank made crowded so we can see the fish to purchase it. The size of the tank does matter, a large tank has more surface area for bacteria to establish the nitrogen cycle and break waste down before it turns water toxic. In a smaller tank this does not happen with a fish producing a lot of waste as there simply is not the room for it to occur.
This results in our fish getting sick from toxic substances in the water, these vary in there degree of toxicity with Ammonia and Nitrite proven to be harmful to fish long term and in the immediate moment. When we subject a fish to a small tank that is simply too small to sustain the fish to adulthood, it is a form of torture really. We can use a human example of keeping someone in a small closet from baby to adulthood. It simply does not work.
Our members having acted on information provided by you, have seen that the species you have is in too small of a tank which is likely producing toxins (Ammonia and Nitrite), Ammonia is almost as lethal to fish as it is to humans, ammonia poisoning can kill a human in a matter of minutes if exposed to enough of it. In fish it binds to the gills and over time, suffocates the fish until it succumbs to it. Nitrite is not as toxic but still dangerous to the health of the fish..Nitrate removal which is the least toxic form of the Nitrogen cycle by products we remove with water changes and or having live plants in the tank.
The fish being sick is an indication of something not right in your tank...The remedy to this is analyze the tank and see if anything shows up. In your case it could well be the tank is not cycled and there is not enough bacteria to break the waste from uneaten food / fish waste down to a safe level...doing water changes daily will help to an extent but the bacteria need to play catch up and multiply to a level that they can convert it. With a small environment and a high waste producing fish, this ay not occur as the bacteria simply cannot multiply fast enough, this results in use seeing this as high ammonia and Nitrites with low levels of NitRates.
The solution to pollution is Dilution is a great way to gauge when a tank is potentially too small for a fish, too small a tank and the pollution will not get diluted, a bigger tank by having more water allows this to occur.
Now onto the fact that is proven in many ways, the size of the fish. A 12" fish will cram into almost any size tank, can it live in that tank though no, it has no way to swim, no way to turn around and is very susceptible to disease. This is what is going to occur in your tank with the Goldfish
, yes it will fit in your tank, BUT it will NOT survive in that tank for any length of time as it will be poisoned by toxic water or by succumbing to basically being squished to death as it has no room.
Am sorry for this long post but it needed to be mentioned...we are here to help you and offer suggestions which is why you came here in the first place. No one likes to have fish die, we do get attached to them, we do however take on a responsibility to provide a care which allows them to live a long life..