Is it eventually impossible to keep a single oranda goldfish in a 10 gallon tank? Does it matter if it has both a hanging carbon filter and an undergravel biological filter cycled by a 550 gpm powerhead while at the same time performing regular water changes every one to two weeks? Any expert and honest advice would be appreciated.

I have hears undergravel filters aren't the best, but that's on a side note. I have many orandas. The question for you is how big is the oranda? If you want him to reach his or her maximum size of course a ten gallon is too small. I have about ten fancy goldies in a 75 gallon two of which are 6 years old black moors . At 6 years old they are pretty big. So if you want to make him more happy and most of all more healthy I would upgrade your tank to atleast a twenty gallon and if you add anymore make it only common golds for color to the tank don't add any fancy until you have a bigger tank than twenty... Weekly water changes are recommended also for goldies. They are quite dirty

Regarding your question, in doing alot of research fancy goldfish need at least 10 gallons per fish. They tend to grow quickly and need more room. For only one the ten gallon tank would ok for a short while, but I would upgrade to at least a 20gallon. You also want to have 10 times the filtration, so for a 20 gallon tank you need a filter that dose 200gph, they are very messy.

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