ok 29 gallon tank 3 - 1 1/2 inch ryunkins, ammonia- 0   nitrates- 0 feeding once a day, variety pellets, peas, blood worms, brine shrimp. the one's top fin kept staying down and now he is  sitting on the bottom. oh and my  temp 76 and the tank has been up for a year. it's like the fish just clamped down.hope this is enough info. does anyone have any suggestions. my other 2 fish are acting normal.

Are you sure the nitrates are 0 and not the nitrites?

I would suggest something as quick and easy as a large water change How much do you usually change and how often? Do you clean the gravel with a gravel cleaner when you do maintenance?
From now on I'd recommend 50% minimum a week. And you could feed them twice a day actually They'll grow better and bigger with more nutrition since goldfish can't store it
So, now test the water again, write down the results (post them if you want) and change up to 70% of the water.