Need Help With Kois?

I have 3 kois 2 are 5 inches and the other is 4 1/2 They are in a 20 gallon tank with gravel, and a two way rock tunnel type of decoration. Is a new tank nessesary? and how fast do kois usually grow? i feed them this koi vibrance pellets from tetra pond
Answers:
Your Koi will grow slowly in that environment.
Koi produce and release growth hormones into the water. When these reach a certain level - for example they'll reach that level fast in a 20 gallon tank - those fishes growth will slow down fast. They'll still grow, but not fast at all.
So you're safe with your 20 gallon tank for now. But not for long. You will need to trade up to a 55 gallon soon, then up to a 100 gallon - then higher - then as the others have said - a pond. To not do so is not fair to your fish.
You do want them to be happy fish, don't you?
You do want them to live their lives as it was meant to be, don't you?
Koi were meant to live in ponds. Big ponds, not your little 500 gallon pond, but we're talking ponds 2000 gallons and up. The bigger the better.
And Koi in ponds grow FAST. Believe me, I know from my own experience. They grow when they have three things -
a) a large place to swim and hunt.
b) lots of nutritious food, and
c) a healthy environment.
Last Fall I had a 3,600 gallon pond installed in my backyard. This past April I purchased three 5 inch Koi. So now they've been in my pond almost 4 months. Well, each of those three fish are now about 16 inches in length - so in four months they grew 11 inches!!
I also bought two more Koi at the end of May. These two were only 4 inches each when they went into my pond. Now after ten weeks, they're each about 10 inches in length.
Like I said - they grow FAST.

And - these Koi of mine would have even grown larger is I had performed all the water changes that were supposed to be done. But I'm disabled, and couldn't do them all. Because of that their growth hormones built up in the water - that caused their growth to slow down.
So follow the advice of others on here - keep your Koi for now - give them a HEALTHY and HAPPY start in life - but know that they really belong in a Pond. Start looking for someone to adopt them - a responsible pond owner. And check out their ponds first. You can tell a responsible one from an irresponsible one by the condition of their pond. Believe me you'll be able to tell! Find someone good. Find someone that will let you visit your "babies" now and then in the future.
And remember that a Koi in a Pond is where they'll reach their full potential as a Koi Fish - and fully grown Koi are beautiful!
Good Luck with your Koi younglings,
Ken

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koi will reach over 36 inches,
so a larger tank is a must.
Really koi dont belong in tanks, try looking into a pond system depending on where you live.
It is a general rule that koi should have 2000 gallons per 2 fish to fully develop.
Remember you can keep them where they are, but they will be stunted, look like crap, decreased lifespan and a overall bad life.
Get a larger tank somewhere over 100 gallons, for a few months then another move, or get a pond. If you cant care for them, try and give them up for adoption to someone who has a pond system.
Koi tend to be alittle bit slower in growth rate, but they will get large and can live for 30 years around.

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You're not supposed to keep the koi in a tank. Rather, goldfish should be in a tank. Koi are better kept in a pond. You should get one of those black basin things where you dig a hole in your backyard in the shape of it and put the basin in. Then you add a pump and filter and whatnot. Koi are appreciated for their "aerial" view, as in looking from the top, not a side view.
By the way, if you're going to make a pond, try to put it under a tree.
Good luck!

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You'd be better off to put the koi in your own pond if you have one or give them back to the place you bought them from. Many people buy koi when they're small but do not realize that they DO NOT stop growing. Even if it seems they are maximum size they'll never stop getting bigger.