Q: Anon wrote,
I want to get another Siamese Fighting Fish but don’t want to have to set up a whole new aquarium just for him and I don’t like the idea of using tiny bowls. I currently have a 10 gallon with 2 fantail goldfish. Can I put my Fighting Fish in with the goldfish or will that put them at risk of being attacked?
A: Bettas and goldfish generally don’t make good tank mates. This is not to say that they have never been housed together successfully but their environments are so different that you would likely be compromising one species or the other. The most obvious difference is that Goldfish like your Fantail are cold water fish and thrive at a temperature between 55 – 70F [12.7 - 21C] degrees. This is much too cold for a tropical Betta fish, which prefers his water at a nice stable 80F [26.6C]. Even if you compromised and maintained a stable 75F [23.8C] you might find your Fantails stressed from the warmth and your Betta sluggish from the cold.
Fish waste can also become an issue fast, especially where Goldfish are present. Goldfish, with their large mass, create a tremendous amount of waste that can quickly reduce the quality of the water. Because Goldfish put such great strain on the bioload it is often recommended that they each receive a bare minimum of 10 gallons [37.8 liters] per fish. As your fantails grow you will likely need to transplant them to a larger tank with a more intense filtration process. Bettas, being very sensitive to poor water quality, can succumb to a host of illnesses if exposed to ammonia, nitrite or elevated nitrates caused by the Goldfish. Being weak swimmers, they can also experience some stress from exposure to heavy currents caused by Goldfish-worthy power filters.
Their ideal pH tends to differ too though both are fairly hardy. Bettas like slightly acidic water ranging from 6.0 – 7.0 where Fantails enjoy more basic water and do best at a pH from 7.0 – 8.0. I wouldn’t say pH is totally critical here as long as it’s stable but again your set up could be favoring on species substantially over the other.
One thing you probably don’t have to worry about is your Betta attacking your Goldfish. They are so much larger I would suspect your Betta would want to leave them alone. Truthfully,I don’t know anyone personally who has housed Bettas with Goldfish so I can’t say with total confidence that they would play nice together.
My advice is to hold off on getting a new Betta until you can set up a proper tank for him. Like you, I’m not a big fan of Betta bowls, but I think your betta would experience significantly less stress in a well maintained Betta bowl then he would in a 10 gallon with two Fantails. If you insist on housing your Betta with your Goldfish I would encourage you to maintain water parameters that average the two extremes and to be especially diligent about your tank filtration and cleaning regimen.
Best of luck.