this is going to be about Sirius Black, my 10yo son's betta. Sirius has been with us since November and he's a pretty happy fish. He lives by himself in a 3.5 gallon tank, has a filter, lighting and live plants; we change out half the water every week and make sure the parameters stay within norm.
Today we came home after a night out and noticed a strange spot on the back of the fish. This is what it looks like from the top:
And this is what it looks like from the side:
I've never had a sick fish before and my kid is freaking out. Can anyone please shed some light on what this might be - some kind of disease, or an injury, or something else? The fish was totally fine in the morning. He also ate pretty well at night so no loss of appetite or lethargy were noted.
Thanks so much in advance.