Unless there is so much C02 it's causing suffocation it's probably not the cause. If levels were high enough to affect breathing, it would affect all fish too. The nitrates are higher than most prefer to have them. Levels up to 50ppm are said to be safe, but most try to keep them below 20ppm. Do more water changes to lower the nitrate levels, and plants can help use it up as well, and if this is a planted tank, with a lot of plants, then I think you may not be doing enough water changes to keep up with the nitrates. How many fish, and what kind, are you keeping in the tank ? I'd keep the fish isolated and observe. It may be sick, it may not be, but the best thing is what you've done already, remove it from the main tank until you can find out what's up. There really isn't anything particular to do with just this symptom as yet other than keep the water it is in very, very clean. If it continues to improve you can return it to the tank later on. If more symptoms appear, post them. The erratic swimming is not necessarily a sign of something terrible, but it's not normal either, so isolation is the best thing for now.
