I'm new at keeping fish... I had a real nice pond all summer but my husband wanted me to bring the fish in for the fall and winter. I have 24 fish in all and 3 snails. 6 of my fish are koi and the rest are different gold fish. The water had turned green and being it is 120 gal tank it is not easy to change the water all the time. Right now I am useing the filter and fountain from my pond to clean and oxiginate the tank. What do I need to do to keep them in the house and keep them healthy and happy?

Welcome to the forum, Mindemae is right, she and Nossie are the most experienced on here I believe 
And the rest are sweet peeps with lots of knowledge too, lol and we all learn from each other by sharing our experiences. I love this little forum 

Anyway, Mindemae is right, the tank is overcrowded and in this case you really need the best filter system you can get. Then the test kit. You will need to perform regular waterchanges, maybe twice a week because of relation tank size -  number fish.
Also very good aeration with some airstones and live plants, as your fishies are used to having them and they love to nibble them... like a salad snack.

I am using an external filter as well If your tank has a stand or shelf under it, you can hide the filter there I have mine standing next to the tank, because there's a wall on the cabinet under xD Anyways, be sure to get a filter that can pump at least 1200 gallons/h. Even bigger than that would be better! External filters can usually handle a larger bio load and a bigger amount of water, so they're usually recommended for tanks bigger than maybe... 60 gallons, I believe. But there are smaller external filters available too

They basically look like this:
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With hoses connected to the upper and lower outflow respectively intake. The one on the picture is the one I've got, but they are all basically the same, just with different looks It's sometimes good to combine filtration in your tank to make it even more efficient! I've got a smaller internal filter as well, rated for a 56 gallon, but combined with the external filter it works very good!
You should try finding a pet shop with lots of knowledgeable people and ask for advice about filtration there!

And I would, if you can, give away a few of those fish. At least the koi, because they will outgrow the tank in a minute! Your 120/125 gallon can only support a maximum of 12 fish. Unless you'd be checking the water chemistry and doing regular water changes when needed. But try to decrease the number of fish to maybe 14-16 or something With good filtration you can pull that off!

That's your choice If I was you, I'd try to keep the koi in the pond at least, because you can't fit so many big fish into one tank, even if you try to change water often and keep the water quality as good as possible, the fish won't have space to swim, and they'll most likely get really stressed in the crowded tank, which leads to illness

You could always get a second tank and keep some fish there to try and bring down the numbers! Nothing big, maybe a 60-80 gallon to keep some of the fish in and you could leave the rest in the bigger tank!

Koi will usually outgrow goldfish and most ponds really fast, and start bullying the smaller goldfish. That's why it's usually adviced to keep koi by themselves, and goldfish by themselves. The koi should easily survive the winter in the pond as well as long as you won't let the water freeze completely. They'd go into hibernation and simply sleep it all through

What kinds of goldfish do you have?