My tank and some thoughts on keeping koi in an aquarium.

Koi Fish Pond Observation Tower - 22 Small Garden or Backyard Aquarium Ideas Will Blow Your Mind
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Can you keep Koi in an aquarium? Well, if they are less than 4" - sure! Koi have evolved to go deep and hide at the bottom when threatened, so sometimes an aquarium - with glass on all sides but one, is quite threatening to them. But when they are babies, it's fine to keep them in an aquaium (with proper filtration, of course!) to watch them grow before putting them out into the pond. Just don't be surprised if you only get a few months before they outgrow the aquarium. Generally, once they reach 4-6", they need to go out, where they can graze on algae and just be Koi! Some Koi keepers don't enjoy seeing Koi in an aquarium because you're viewing them from the side, while Koi patterns have been developed to be viewed from the top down. They sure look different from the side, and that's a learning opportunity too! If you do decide to keep them indoors with you for a bit, check out these awesome aquarium designs - I WANT ONE! Click on the picture above, or the title to see all the pictures...
Koi are pond fish and bottom feeders and don't belong in an aquarium.
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I have seen many articles about raising koi in ponds, but I have never seen anything about raising them in ordinary fish tanks. I have six koi, each several inches long, and they currently reside in a 125-gallon tank. The tank has an external canister filter and an undergravel filter. How long is it safe to keep koi under these conditions? Would it be possible to keep them outdoors in a pond but bring them indoors and keep them in the aquarium tank for viewing for several months? So~ if your looking to keep koi in an aquarium, reconsider...maybe goldfish is a better choice.
Photo provided by FlickrTiger pleco Koi & yellow, white long tail butterfly koi fishes in an indoor aquarium.
Photo provided by Flickr“There’s a place for koi in the world,” Shepherd said, “but it may not be at Steinhart Aquarium.”
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Those who keep koi fish know how delicate and important it is to maintain their living environment. As with every fish, an aquarist should understand all the basic necessities and requirements of koi fish.Always make sure your water is properly filtrated, well oxygenated, and all waste is cleared from the environment. Be aware of your fish’s exact needs at each time as they can change due to number of fish, maturity, and temperature. Expert aquarists advise koi keepers to use high-volume filters to save time and effort. Ideally, your pond should have a bottom drain for easier cleaning and water changes. Hi; Nishikigoi in sm. aq.s 9/1/13
I just bought an Aqueon 15 column aquarium and have 5 small koi's in there and I don't know nothing about fish plz help.

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Bob Fenner>If you are going to keep Koi in an aquarium, you want to make sure that you have very, very good filtration; strong, fast-moving water, highly-oxygenized. You want to keep the pH between 7.2 and 7.6. They like slightly basic pH. You want to feed them very good foods, vegetable-based foods. You also want to maintain the water temperature where they need it to be so their immune systems can function properly. They do not fare well in warm water, so it’s best to run a chiller on a Koi aquarium. You want to keep the water temperature to low to mid-60’s. That is where I feel they do best. The oxygen levels will also stay higher by keeping that water temperature a little bit cooler.To keep Koi fish in an aquarium, you want to try to replicate their natural environment, just like you would do with any other fish. Koi fish require very clean fast-moving water, highly-oxygenated water, and they need a very large environment. They get very, very large. Even though freshwater fish, their size will be limited to the size of the aquarium, their growth will be very stunted; their gill plates will keep growing, their fins will keep growing. They’re going to get a hunchback appearance to them. You want to go with a very large aquarium. I don’t recommend Koi for a small aquarium; when I mean small, anything less than 200 gallons. The Koi will just outgrow that, they’re not going to be happy in there.Koi are cold water pond fish, descended from carp. Koi owners may need to bring their prized koi indoors during the winter to prevent death in frozen pond water. Keep koi in large aquariums for a few months at a time. If you live in an apartments, you may have to keep koi in aquariums for their entire life. That is possible, as long as you do not overcrowd the aquarium with too many koi.