There are 4 fishes there, 2 gold, one white and the small one (that's sick). The two gold fishes have been there for 2 years. Around 1 month go I got the other 2 (the white and the small). Never had any problem, this is the first time something is happening. The tank is around 31cm x 26cm, and 26 cm tall. It has a small filter that runs 24/7. I talked to the guy who sold them to me, he said it's kind of "paralysis" that happened rarely. He said that it happened on my shop tank too and there's nothing you can do but just leave it. He said the fish is not suffering , and it will continue to eat normally . Any suggestions ?