Not to mention the fact that goldfish can grow to a rather large size. And the tank could literally be too little room for both your gold fish and betta. Okay, okay, sure the gold fish may be stunted and all living in cramped quarters. But it that really good for it?
Point is. Bettas and goldfish are the worst match possible. Like keeping chocolate in your pocket on a hot day. Or big hair in a hurricane. Or doing xBox with a runny nose. Disasters one and all.
Goldies and bettas don’t mix well either. Despite what this video appears to show.
Okay so if this is not enough to convince you, let’s find out some of the real things gold fish and male bettas disagree about.
First, bettas and gold fish have different temperature requirements. Goldfish are cold water fish so they thrive in water at 50-60 degrees Fahrenheit. Then again bettas are tropical water fish so they thrive in a temperature range between 72 and 85 degrees. Otherwise putting your betta in a cold water environment will just about guarantee sooner or later you’ll have one sick betta fish on your hands. While putting a goldfish in warm water may allow them to survive this will not make them thrive. So while you wouldn’t think to install a heater in a goldfish tank, a heater is a must in any betta fish tank.
Second, bettas are "obsessive compulsive." Any amount of toxin in their water can leave them sick and dead. You need to ensure their water is clean at all times. With that said, you need to constantly monitor the water’s pH as well as its levels of ammonia and nitrates. Now if you put goldfish in a betta fish tank, they are known to excrete a humongous amount of waste toxic to bettas.
Since goldfish are (ahem) what you might call heavy poopers. So you need to install a strong and powerful filter in their tank. Now the bettas, seeing’s they go heavy on the fins may not be able to withstand such strong currents. It’s too stressful for them. Either the current could damage their delicate finnage or they might find it difficult to swim through heavy flow of water. Although it’s still possible to place a filter in any betta fish tank especially if you’re having a cycled tank, but you can always look for ways to somehow control the flow.
Third, goldfish could grow as much as one foot long so if you’re only putting up a 5-gallon tank, there wouldn’t be a room enough for the goldfish to swim around much less share some space with your betta. The goldfish could also pick on the tiny betta and you wouldn’t want to see your betta with tattered fins.
Lastly, bettas and goldfish feed on different types of food. While goldfish are vegans, bettas are carnivores. And even if you throw different foods into the tank, the goldfish may still outcompete and elbow aside your betta. Leaving them hungry and cold.
Perhaps now you see why bettas and goldfish are the perfect mismatch. So don’t push it. Instead, try to look for other fish that are more compatible with these somewhat finicky bettas – if you need to put any other fish with them at all.