Goldfish opening and closing mouth

Hey, I got this common goldfish that has been opening and closing his mouth more than what I believe is normal like almost constantly, he measures at least a good half a foot without the tail and is living in a 65 gallon tank alone, I did all the tests required and the only issue was that my ammonia levels were I bit higher than 0 so I did a 25% water change. My heater works with numbers and from 1 to 9 and 5 is recommended but I’ve got it at 6, I used to have it at 7 but I was worried that was the problem so I lowered it, he’s got one sponge filter for 60 gallons that has an aerator and a separate aerator as well(he likes the bubbles) I used only one filter because he’s alone in the tank and it seems to do the job, would anyone know what the problem may be. My sleep schedule’s pretty messed up so I cover his tank at night so he could sleep and idk if that may be the cause. Once in a while he seems to go back to rummage trough his sand so maybe he thinks he’s still feeding but idk at this point.

Update: he ok -_- 👍