Bettas are known to be aggressive but goldfish are similar in size and actually grow to be much bigger than Bettas so they’re likely to stay away.
Goldfish, on the other hand, like to nip at fins which would agitate the Betta who also likes to nip. It’s just not a good combination.
Cleanliness and Filtration
As far as cleanliness goes, these two fish couldn’t be more different. Bettas are generally clean and don’t respond well to dirty water with high ammonia levels.
On the other hand, goldfish produce a lot of waste which can quickly cause ammonia levels to rise.
Goldfish require frequent water changes and a pretty powerful filter to keep the ammonia levels down. Frequent water changes are actually very stressful to a Betta and they prefer a gentle filter to one with high output.
Water Temperature
One of the major difference between these two fish is that Bettas are tropical. They need water that’s between 78 and 80 degrees F.
Goldfish prefer colder water no warmer than 75 degrees F and even like seasonal variations that dip to the 60s in the winter.
A goldfish would not survive in the warm temperatures that a Betta needs and vice versa.
If a goldfish is kept in water that is too warm, it will become stressed and more prone to parasites, illnesses, and other conditions.
One of the most common is Swim Bladder Disease when they actually swim upside down which leads to exhaustion and other health issues.
Bettas kept in water that is too cold also become stressed and prone to parasites and illnesses. Their activity also slows down significantly and they’ll be more and more likely to just sit around the tank instead of their usual flurry of activity.
Habitat
Even though they’re sold in tiny plastic containers in pet stores, Bettas should actually be kept in 5 to 10-gallon tanks with a large surface area. The actually gulp air from the surface from time to time.
As for goldfish, it’s a common misconception that they can live in a small bowl. Goldfish actually require a lot of room, ideally 20-gallons for just one goldfish.
They’re meant to grow pretty big – some can reach a foot long! – and need a lot of room to reach their full potential.
Betta Fish and Goldfish: Not a Good Match
You might have heard stories or seen videos of Betta fish and goldfish living together and acting like the best of friends but there are a lot of reasons why these two fish cannot live in the same tank.
Both species can be aggressive at times and it’s likely they’ll nip at each other’s fins. Eventually, the goldfish will grow significantly larger than the Betta and get the upper hand.
In addition to their incompatible temperaments, they’re basically complete opposites when it comes to everything else.
Bettas like warm water, goldfish like cold. Bettas are clean, goldfish are messy. Bettas like calm water, goldfish require a powerful filter. Bettas like 5 to 10-gallon tanks with a large surface area, goldfish require at least 20-gallons.
It’s not easy to find a tankmate for a Betta and, truth be told, they’re often best left alone. That said, there are some other species that are a good fit with a goldfish. By finding the best tankmates, everyone will be happy.
There are a lot of reasons why Betta fish and Goldfish shouldn’t live together. 
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