somersaulting goldfish

Question
I got three fantail goldfish 1 1/2 yrs ago.  About 6 months ago one seemed to stop growing, and one seemed to accelerate growth.  The smallest developed problems swimming and seemed to be missing a tail fin - he never grew it.  He lost his gold color and started wedging himself vertically into corners, nose down. Eventually my catfish got him and he died.  I thought his problems were due to his deformed tail.  But now the two larger goldfish who had been normal are doing the same thing.  The largest wedges himself upside down and stays like that for 10 minutes at a time.  They don't float to the top or have trouble staying submerged.  They just slowly do forward somersaults if they slow down swimming.(I removed the catfish)  The tank is 30 gallons, with a bio filter. they are the only two fish in it, & I do regular water changes, 20% every week.  Nothing has changed except their behavior.  I stopped feeding goldfish food & started feeding frozen, microwaved peas as recommended on this site to other people whose goldfish had similar problems. I started that three days ago.  No improvement yet.  Can you give me any further advise?
Thanks so much!  

Answer
Hi Susan;

The peas should correct it if the problem is due to a digestive issue. You might try fasting them for 3 days first and then offer peas. This means no food at all for 3 whole days. Then try peas again.

If your goldfish are fantailed types with roundish bodies, sometimes they have trouble with their swim bladders for life due to a deformity in it's development. The rounded bodies have caused them to be too compacted internally so the swim bladder never really forms properly as they grow. Here is an article about the swim bladder and it's function;

http://www.fishtanksandponds.info/care-maintenance/health/swimbladder.htm

I hope they feel better soon.....

At Your Service;
Chris Robbins