I would say no. They are strong enough to break the glass, and they would get 2 feet ish. I would say no, koi need ponds.Would it be possible to keep koi in a 5ft x 4ft rectangular aquarium that was 3ft/4ft deep? I've no idea how much water it would hold or how to work that out.
1 koi needs 500 gallons of water, but I don't know the footprint though.
You could keep them in there until they are around 30 cm long then move them into a pond. Just make sure you have a lid I had a small koi of mine jump out of the tank and pass away, so lids are a must.Would it be possible to keep koi in a 5ft x 4ft rectangular aquarium that was 3ft/4ft deep?
EDIT: Aquarium calculator says this would be roughly 2,250 litres