Swelling/bulge on my Bettas face
I need some advice about my betta. I’ve had him for almost 5 months, and this week I noticed the left side of his face/gill area looks swollen or like there’s a bulge forming. He’s still swimming around and has a big appetite, but I’m worried because it definitely looks uneven now. I attached a few photos from different angles to try to show it better. Does anyone know what this could be or what might cause it? He’s in a planted 10 gallon tank with stable water parameters (API test kit shows everything in the safe range), the tank stays at 79°F, and I do water changes every 1–2 weeks depending on the tank. I’ve also added more tannins/catappa leaves to help reduce stress and soothe him, but I’m not sure what else I should be doing. Any advice would really help, thank you!