What size tank is he in now?
Filtered? What type?
Do you test your water parameters? What method? What are the results?
How often do you feed him? How much? What food?
Answer the questions about your decorations.
Separate the betta and the goldfish. Besides the fact that the betta needs WARM water and the godlfish needs cooler water, the goldfish is a poop factory and will foul your water before you have time to blink.
Do a 50% water change. Plan on doing more frequent partial water changes. Melafix should help, but only use 1/5 the dose on the bottle as it is concentrated and bettas are sensitive to the ingredients. You can buy BettaFix, which is the same stuff already diluted for a higher price.
Bettas are solitary creatures and don't need the company of another fishie to be happy.
They need plenty of clean, warm water & regular feeding of a variety of foods.
Check back in with the answers to these questions and people with more experience will be along to help.