Your goldfish has a swimbladder disorder. More info;
a. Swimbladder disorders are often caused by internal bacterial infection, this often starts with constipation but sometimes it can purely be just the internal bacterial infection and no constipation.
b. In fish like fancy goldfish (like orandas), swimbladder disorders are most commonly caused by constipation. Many fancy goldfish are already prone to this condition due to the way they have been bred to have a very compact and unnatural gut shape (unlike "slim type" goldfish like common and comet goldfish which have a more natural body and gut shape), they guts are prone to getting constipated because of this fat compact gut shape and when they do get constipation this puts pressure on the swimbladder, causing the fish to have a swimbladder disorder. In longer term cases of untreated constipation, the food can start to rot in the fishes gut, usually causing a bacterial infection and gas problems which makes the disorder much worse and more difficult to treat.
c. What causes constipation? Over-feeding or feeding too many dried (particularly high protein foods) like fish flakes/pelets or dried foods like tubifex and bloodworms and/or not feeding enough veg in the fishes diet.
Basically, the most effective way to deal with swimbladder disorders is to deal with the fishes diet and treat the fish with a med that specifically treats internal bacterial problems (like "anti internal bacteria" by Interpet). To deal with the fishes diet you should cut back a great deal on the feeding of dried foods (even temporarily stop- don't worry the fish won't starve, goldfish can easily go up to two weeks without food, goldfish have a lot of body fats which can allow them to go for long periods of time without much or any food- its how they hibernate in ponds during the winter), and feed only veg (like chopped up cooked de-shelled pea's or blanched or cooked and chopped up spinache) and feed only frozen foods like daphinia (which is good for fishes digestion), which you should be able to get from any good lfs/local fish store.
If you do this, and keep the tank clean, the goldfish should be better in no time
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I would definately advise you do water changes on a more regular basis, more like 50% once a week.
The average tank cleaning regime for anyone's tanks (as long as they are not cycling or overstocked), should be like this;
a. 25-60% water change once a week with dechlorinator.
b. Clean substrate once a week.
c. Clean filter once every 1-3weeks or more (depending on how quickly it gets dirty, you should clean it often enough so that it has a decent current).
d. Check heaters, filters, lighting etc for any limescale buildup, check to see if everythings working etc.
e. Test tank for ammonia, nitrite, nitrates and ph every now and then to make sure there are no water quality issues- even if the water looks sparkling clean, you can't see stuff like ammonia and it may be present in the water- the only way to find out if your water quality is good is to test it.
In a healthy tank ammonia and nitrites should be 0, with nitrates preferably kept under 40.
I have to warn you that for a 10gal, its too small for your goldfish. For fancy goldfish, you should at least 20gals for the first one and 10 for every one after that. At the absolute bare minimum, for three fancy goldfish they should be in an over-filtered 30gal long tank. Deseases/illness in fish is also commonly brought about by stress too, like from inadequate water quality or overstocking the tank etc etc, so it is in your fishes best interests and health/quality of life that you upgrade their tank to help prevent encountering future health or behavioral problems with them
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