2 Koi and a fantail goldfish in a 20 gallon tank?

  • #1

I just recently joined, and I am not a fish pro.

I have a 20 gallon fish tank currently occupied with a single tiny fantail goldfish. I am about to be moving soon, so I was thinking of rearranging the inside of the tank and making it mostly "asian" themed with bamboo decor and all. Afterwards I would then go out and purchase two koi to go in the tank along with my cute little goldfish, Jackie Chan.

I have read it is fine to put koi and goldfish in the same tank together, however, I question if this tank too small? Also, after buying all new products would it be best to allow the tank to mature before buying the fish or returning Jackie Chan to it? And for how long?

What other type of fresh water critters could I fit in there that would benefit the tank?

Thank you for your help
  • #2
I hate to give the bad news but the koi will not fit in a tank that small. Koi grow to be very large and need plenty of space to swim around. Most koi are placed in small ponds in a back yard or very large indoor aquariums. You can read more at

As far as you fan tail I don't know how big he/she is supposed to get but the general rule is one inch of fully grown fish per gallon of water. So if you fan tail is going to get to be 8 inches you can put maybe two more goldfish in with him but need to check compatibility with other goldfish species.

You can not mix tropical fish and goldfish because of the temp requirements. Tropical need much warmer water and will not be able to handle the cooler temps that the goldfish need. But never fear there are plenty of goldfish species to choose from just reasearch and choose wisely
  • #3
Koi grow huge. Goldfish grow big too, but they produce so much more waste the inch per gal rule doesn't really apply. You shouldn't keep more than one goldfish in a 20gal.

Do you know about the nitrogen cycle?
  • #4

When I usually search for info on fish I usually don't find to many sites talking about bio-load or the amount of waste produced. Am I missing that or is this something you just get to know by experience?
  • #5
Idk, you just learn stuff between research, experience, and consulting people on forums...not many sites out there are as good as fishlore so it can take some time to find specific info.
  • #6
Welcome to Fishlore and yes, the tank is too small.

Goldies are heavy waste producers and they grow to large sizes so even a 20 gallon tank is small for 1 of them. Jackie Chan can live a good live in a 20 gallon tank, but he would be in goldfish heaven if he could get upgraded to a 55 gallon or larger in time. Then you could give him other coldwater tankmates.

Koi are magnificent fish but unfortunately would need at least a 200-350g tank for just 1 and they are better kept in small ponds as they can get very big and live a LONG time.
  • #7
Omorrokh and Dave's message, and Guppy's first paragraph are all right. There have been koi recorded as large as 4.5-5.5 feet long, not counting bulk. There are also koi that have been recorded to have lived over 200 years. (most only live 20-30, but there are some that live incredibly longer than that)
  • #8
You'll definitely want to let your new tank cycle when you get a new one for your fish.
This link will give you an idea of how to do this.

Your goldfish will be happier without any tropical fish, as goldfish prefer changing water temps (the statement that they are a coldwater fish is a bit of a misnomer, they can actually handle much higher temps than tropical fish can. What they don't like is a single, steady temperature. They like going through "summer" and "winter" times)
This does not mean that your tank will be limited to only goldfish. If you provide plenty of hiding spaces, there are shrimp that do well in changing water temps, like the amano shrimp. Some snails will do alright, as well. There are people who have kept crayfish with goldfish, but there is a danger that the crayfish will kill the goldfish. Frogs and crabs present another possibility, just do your research on size, diet, and temperature requirements.
  • #9
Excellent points SDS! The temp. range of comfort for goldies is pretty vast. I believe 4-27c whereas most tropicals have a very small range of comfort.
  • #10
Could you translate that into real temperatures? It doesn't matter if the rest of the world uses that little imaginary temperature system. They should be using ours. (Sorry, I'm channeling the US delegate to the world climate summit right now )
  • #11
4-27 celcius is: 39.2 farenheit to 80.6 farenheit.
  • #12
That was meant to be tongue-in-cheek, but thank you for the link.
  • #13
Could you translate that into real temperatures? It doesn't matter if the rest of the world uses that little imaginary temperature system. They should be using ours. (Sorry, I'm channeling the US delegate to the world climate summit right now )

I thought I was just old and set in my ways!!! Having the computer really does help me see things the way they really are...
and convert where necessary.
  • #14
Welcome CarpQueen, to fishlore.

I love your idea of the new home for Jackie Chan, but have to agree the idea of adding Koi would just lead to heartache.

I agree that shrimp might work, if you could find some that get big enough not to be an appetizer for Jackie, and could withstand his temperature range. This would add lots of interest, and stick with your theme.

The one thing you will find here is we all promote MTS, "Multiple Tank syndrome", and I agree you and Jackie would both be happier with a 55 gal and a couple of goldfish. My LFS just got a new one in for their display tank. It is a 180 gal, and has two 5" fan tail goldfish, and just added one that is solid white and about 7 or 8" ABSOLUTLY FABULISE. It rivals ANY Koi I have ever seen! The fun of having only a few fish in a really large tank, and seeing them grow to their potential, and in good health, is all worth it!

Let us know how it goes!:;rudolph
  • #15
koi get very big so they aren't advised for an aquarium. But you can add more fantails to your tank. I personally love the ryukins because of their calico coloration. Also, get them small. They will grow and I think they are uglier when they get big and super fat. Also, they are much cheaper when they are smaller. There are many other types of fantails too. Moors are an asian fish if you are going for that look. They are black fantails with bugeyes.

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