So I have had my ryukin for six months now. I had him in a larger bowl for the first five months. The bowl was actually about 15 gallons. There was a air pump in there for circulation, two plants and I also cleaned it out once a week.
I have recently move him to a fish tank. I think it's 13 gallons (kind of an odd size). I have him with another new baby ryukin. I'd noticed my older ryukin floating upside down three or four times. The first time I yelled in surprise and he heard me and floated away. The other two times he was floating upside down and my boyfriend saw him and poked him lightly to make sure he was not dead. I read online this is because of the swim bladder and that peas can fix them.
Let me add that my ryukin is missing a pectoral fin, so he generally likes to sit at the surface of the water, waiting for food, like a shark. It's very cute. I think he was getting too much air when he was eating... so I got a pack of frozen peas, thawed them out, took them out of their little pea skins, crushed them up and fed them to him. Now the water is horribly diluted with peas. It's been five days since and now he just lays on the bottom of the tank. Every once in a while he'll get excited, swim a little and sink.
I now have a filter pump that says it's for 10-30 gl tanks attached to the tank. I've cleaned the water out TWICE and have had the pump filtering for 2-3 days. The water is still as murky as ever and I don't know what to do.
Here's a video of my babies before the one ryukin was sick! I love them so much

Check out the calico's missing fin. I think it gives him character