Betta Fish: betta fish swollen gill?

i am a fairly new betta owner. i have had him for 2 months. yesterday i went to feed him and saw that his left gill is swollen/has a bump on it. i immediately changed 50% of the water. i only have silk plants and nothing sharp that would hurt him. i dont have any live plants. i change his water every one n a half weeks. is this a tumor? i just ordered a new test kit to test for ammonia and nitrites and it will arrive on monday. can someone advice me what on what to do if the levels are off? its been fine so far and he has been healthy with no issues. how do i know if its an infection? and how do i treat him