Your betta is pretty - seems like you have him happy and under control.
How are the goldfish doing?
I used to have some really cool Black Moor goldfish years ago - loved those guys!
Goldfish usually like cooler water so sometimes you can get away with no heater, or if you have a heater you can set it pretty low.
Goldfish tend to be pretty 'dirty' - they generate a good bit of poop etc...the ones I've had eat quite a bit too (fish after my own heart - haha). So a good filter is needed.
What kind of Goldfish are they?
If they are smallish you can keep them in that small tank and change the water a LOT - but you will need to plan on getting a bigger tank depending on what kind they are and how big they'll get.
The bigger the tank - the easier it is to cycle and maintain - also more forgiving if something gets weird with your water chemistry (counter intuitive I know - but true).
I'd love to set up our old 45 gallon tank and make it a goldfish tank. Maybe someday.