Don't let their peaceful behavior trick you; goldfish are plant killers.
Bettas and goldfish love interacting with people, but that's where their similarities end. Place them in the same tank, and the betta will probably pick a fight with the goldfish. Of course, if the temperature is to the goldfish's liking, the betta won't live long enough to throw down.
Don't let the angry appearance of a betta fool you into thinking they're tough, hardy fish, at least as far as temperature is concerned. They a tropical species and prefer temperatures that range between 75 and 86 degrees Fahrenheit. Anything outside of that range can result in severe stress and death. In contrast, goldfish are usually considered cold water fish, thriving best in temperatures that range between 65 and 75 degrees Fahrenheit. However, these commonly kept fish won't keel over in other temperatures. Texas A&M University Southern Regional Aquaculture Center explains goldfish can tolerate 95 degrees Fahrenheit water temperature and 32 degrees Fahrenheit temperature. Simply put, you don't need a heater for a goldfish, while you do need one for a betta.
As carnivores, bettas aren't too fond of plants, preferring to chomp down mostly on meats. They require more protein in their diet than goldfish, which are omnivores and enjoy a nice mix of both plant material and meats. Feeding goldfish betta food or vice versa will result in poor health for your fish. Because of their feeding habits, goldfish sometimes pick at and eat certain plants in your aquarium. Bettas enjoy their plant friends and rarely touch them.
Bettas don't grow very large, usually less than 5 inches, depending on the species, but that doesn't mean they can be kept in a tiny bowl. They love to swim around, explore and float around in large bodies of water. You'll need a 2-gallon tank at the very least to house the colorful and tropical fish. A 5- or 10-gallon tank would make your betta even happier. One goldfish needs a minimum of 20 gallons, while additional goldfish require 10 gallons per fish. Not only do goldfish grow larger than bettas, but they also produce significantly more waste, largely because they have no stomach.
Bettas go by one rule: almost anything that swims is an enemy. There are exceptions, such as certain types of tetras and otocinclus catfish, but bettas attack most other species of fish. And they certainly don't like other bettas. To overcome this problem, you can divide a tank in half, blocking your betta from other fish. Goldfish are far more docile and swim blissfully alongside most species. However, they are omnivores and it's usually not a good idea to house goldfish with fish that can fit inside their mouths.