Can a koi fish be kept in a tank?

If you're going to get koi you're going to need a pond down the road. You're right, they are indefinite growers, they can also live to 75 years old so you need to do some serious research before deciding you're going to get them. The rule of thumb for fish is 1 inch of fish per gallon of water, so if you have a 20 gallon tank you can have 20 1inch fish or 5 4inch fish or 4 5inch fish, you get the idea. But generally you should buy the tank big enough for the fish for when they're done growing and since koi can be a couple feet long you might want to find a bigger tank. Temperature for koi is usually room temperature but if you get a heater and set it to one temperature, maybe 70-75 degrees, that will keep the tank from heating up too much or cooling down too much which can happen when seasons change or the front door is opened too much or if the tank is too close to a heat source. As for decorations, that's up to you but usually freshwater fish owners put gravel or rocks in the bottom and a few big pieces of ornaments or rocks or whatever for shelter. Make sure you get an air pump and a filter, koi tend to make a lot of waste and will need regular water cleanings, usually once a week is good. Make sure to cycle the tank before you put the fish in, so fill it with water, put in the water conditioner (yes, you need this too, otherwise the fish will die) and a pinch of fish food and leave the tank alone for a week. The reason for this is you need to establish some bacteria for the fish to survive. When cleaning the water you'll need a gravel vacuum and a bucket, pretty much all you do is syphon out 25% of the water and mix new water (with water conditioner) to go back in the tank. Snails will help clean the algae off the tank but you'll need some algae before you can add snails or they won't survive since that's their main diet. Do some research online or check out aquarium books at a book store, that should help answer any questions you have. Or you can email me if you have anything else you want to know! Happy fish keeping and good luck!