Okay, losing control of the buoyancy can be caused by a number of reasons:
1. BAD WATER QUALITY. The most common one and happens when people keep goldfish in too small, overcrowded or underfiltered systems. If your oranda is in a tank smaller than 20-25 gallons/80-100 liters, and/or you're not changing the water and cleaning the gravel once a week, this is most likely the reason she's sick.
2. BAD DIET. If you're not feeding her vegetables and live foods along with the flakes and pellets, she may have an issue with her digestive tract. Usually it helps if you avoid feeding her for a day or two and then give her only peas for a few days until she improves A dose of Epsom salts will also help, but be sure you change out 50% of the water and vacuum the gravel thoroughly before dosing! You can dose Epsom salt by adding 1/4 teaspoon per gallon of water. Dissolve the salt in a bucket with tank water and then add it.
3. Water pressure. If the fish is kept in a tank that's rather deep and tall than long and wide, the goldfish may not be able to  handle the pressure on the body and may need the water level be lowered by a few inches.

May I ask how many other goldfish are in with the oranda?

What are the measurements of the hexagon tank? I.e. Height, Width and Length, as it could also be water pressure as Nossie suggested.

There are five fish in total and the others are fine .

It's a tad crowded then, but I'm pretty sure it's the pressure from the water and the lack of a varied diet.
So like scrivens said, avoid feeding the fish for a few days and then buy some frozen peas, thaw them, pop them out of their shells and give them to your fish. All of the fish will probably be very happy for some veggies and peas are generally very good for fancy goldfish
Try feeding them these at least once a week, more is usually better since vegetables are very healthy for goldies. Try squash, spinach, cucumber and some soft types of lettuce as well.
You can also buy some blood worms, tubifex or shrimp to feed the fish once a week for some extra protein, it's good for the oranda's headgrowth (Here's a link to more guidelines on how to feed fancy goldfish: (Link is not visible to guests. Please register to view.) )

I hope the oranda will get better