My Nearly Four Year Old Opaline Gourami is Having Trouble Staying Upright, and is Staying on the Surface

So I have this Gourami who's been with me for about 4 and a half years. He's been good, but recently I've noticed that he's been just kind of floating at the stop. Not gasping for air like some labyrinth fish do when the water quality is poor. His mouth is usually under the water, but his head, or whatever is usually surfaced and, looks a little red.

He usually just sits in one place, and floats. If he wants to turn around or something, he'll often fall on his side, struggle a bit, and then get back on track.

I don't know if he's old, sick, or just something stupid he's doing. I don't know if I should be worried. Water conditions are all 0ppm with the exception of Nitrate which is 5ppm (water change just a few days ago). He's been acting like this for the past month and a half. The tank is a 40 gallon planted.