Strange Goldfish

Hey!

I have a question for you. My chocolate oranda has been acting weird for weeks now, as if he's got a swim bladder disorder (mind you, he didn't do this even once while living in the 10 gallon with 5 other fish....crowded, goddamn crowded). I'm feeding them a variety of food, high quality staple pellets (3 kinds) along with live foods in the form of freeze-dried blood worms and tubifex plus brine shrimp in gel, and occasional veggies and fruits, dried seaweed included.
I use to change the water and clean the internal filter every two weeks and the external one will be cleaned every 4 weeks or so. I'm testing the water weekly and it always comes out flawless, with a pH of 7-7.5, ammonia and nitrite on 0 and nitrate ranging from 10-20. (10 being the week after water change, and 20 being the week when changing it) In other words, it can't be the nitrates affecting his swim bladder either.
I doubt it'd be any infections since the water is kept in top condition and the fish are fed with a nutritious diet. What else could it be?? Especially since he wasn't displaying this kind of behavior ever before.
For more info, the tank's an 83 gal (317L) having around 669gph (2500L/h) worth of filtration. I'd be happy for any help!!

 

Acting like there is a swim bladder problem involved. He looks like he's completely losing control sometimes and just floating around. Sometimes lately even upside down :C I had him separated in a big basket in one end of the tank a few days ago to avoid feeding him for a while and see if it helped, when I let him out 2 days later, I gave peas to everyone, and he acted like a new fish, all perked up! But the next day he was the same again. And the funny thing was that I tried feeding only peas and tubifex for the two first days!

 

Alright... I'll see, and get back with updates!

 

After keeping them without food for five days, I fed them all peas, and today morning i fed them brine shrimp in gel.
Claymore is right back to floating after eating the brine shrimp.

 

MB: In the smaller tank? Mainly pellets, blood worms and tubifex from time to time. There hasn't been any changes in the diet since they moved |':

Anthony: M'kay, I'll try more peas then d: But the last time he was constipated, obviously because of a bad diet, I fed them worms and peas for a few days, and that worked wonders! But now even if I try with worms and peas only again, it doesn't really work |:

 

Thank you so much! Where can you buy this kind of salt? And will it be harmful to the plants in any way?

 

Okay, thanks everyone
I'll be trying with peas for a while first, and if that won't work, I'll go looking up some salt and treat him separately in the quarantine tank