What are the best aquatic plants for koi?

I have four small koi in a 3ft 120 litre tank. Two are about 15cm and the others 8cm. I would like to know which would be the best aquatic plants to put in the tank that they won't uproot from the gravel or eat within a week, or should i just stick with gravel only?

What are the best aquatic plants for koi?
Well, first, for even the size they are now, you need to allow about 38 litres per fish, so your tank isn't big enough. And these are small fish still. When they get full size, you will need to allow about 114 to 152 litres per fish. That's minimum and it should be more for koi. They're pond fish. Anyway, no, don't just stick with gravel. It's not very attractive and Goldfish and koi love to eat live plants. And that's half of why you put plants in; as a nutritious supplement for the fish. They will eat them and you will have to replace them from time to time. They also love to play with plants, so again, they're gonna pull them out. Accept that if you want live plants, you will have to replace them ocassionally.
Reply:Why in the heck do you have 4 koi in that size tank? There should not even be 1 koi in there. You do know they grow huge right. Not suitable for a fish tank, they need to be in ponds.
Reply:Koi love to eat plants and their roots. I would just stick with the gravel in an aquarium. Otherwise it will be a constant battle.