White bump beneath the belly

[usakot]

Our lovely male betta fish, Patrick, has developed a bump beneath his belly over 4 day when we were on vacation. Seems like there is no explanation for this anywhere. Anyone has any idea what that could be? Your help is higly appreciated!

About aquarium:
We got him as a present in a cheap Petco 1 gallon aquarium, not filtered, aerated. Water changes about once a week. He has two guppies living with him, they seem to be perfectly fine.

Thank you!

[xShainax]

The guppies need to go first of all. A one gallon should only house a betta.

[Alcemistnv]

Originally Posted by usakot

About aquarium:
We got him as a present in a cheap Petco 1 gallon aquarium, not filtered, aerated. Water changes about once a week. He has two guppies living with him, they seem to be perfectly fine.

With a one gallon, you should be doing about 1 100% and 1 50% change a week. Also, there shouldn't be anything else in the tank besides the betta. With the small tank and three fish, the ammonia is building upp faster than you think. I believe the minimum for bettas and guppies was around 5+ gallons but someone could correct me if I'm wrong.

As for the bump, I would say fungal, but that's my thought. I'm not very experienced.