Betta Fish: Tumour?

Hey folks, Uth Duna has had a lump on the side of his stomach area for the better part of a month now. He's pooping regularly and I've treated him for bloat and constipation in the past, but it hasn't gone away and has grown slightly. I'm leaning towards it being an internal tumour largely because he's acting completely normal otherwise; he's active, eating and hunting, not flashing or breathing heavily. He has some very mild swim bladder issues that seem to come and go spontaneously i.e. when he stops swimming his back end sinks slightly, but otherwise seems perfectly healthy. Just wanted to get other opinions on this. Thanks in advance!

He's in a 20 gallon long, only tankmates are Malaysian trumpet snails, ammonia 0, nitrite 0, nitrate ~20, PH 7.6, heated to 76f, with a sponge filter. I do 20% water changes once a week. I've had him for about 8 months, he's been in this tank for maybe 6 months. He has all real plants and driftwood, the only artificial things are rests like leaf hammocks and a floating log. He eats King British Tropical Mini Pellets, Hikari Vibra Bites and Fluval Bug Bites Tropical Formula (I cycle between them for variety) twice a day, two or three mini pellets worth, with occasional treats of freeze dried daphnia. He has a frozen bloodworm on saturdays and fasts on sundays.