It sounds like your problem is the lack of food. Little goldfish can survive on mosquito larvae but fish that are 8-10 inches long need to eat more. Koi have hardy appetites. I feed mine every day and they grow about several inches per year. Just be careful not to over feed. Give them the amount of food that they can eat completely in 10 minutes. If they eat it faster, they need more. If there's some left after ten minutes, then you've given them a bit too much. Make sure you have a good filter working since feeding them more means more waste (poop) so your nitrate levels will go up if not filtered. Also, fish eat less when the weather is cold in the winter and more, when they're more active in the hot summer. I'm sure they'll be huge in no time! By the way, I bought most of mine several years ago at 3" and they're now a healthy 12"-16" in length!

If you want them to grow, you need to feed them. My koi get fed several times a day on slow days (days when I am not home) to as many as 6 times per day on the weekends. It's easy to grow koi from 5" to 16" over one summer. In order to feed the amount of food that koi need, you must have proper filtration. Plants alone are not enough for koi. If the fish are 5 years old and only 8" to 10" long, they may never grow well. They may have been stunted too much in their early years. Right now, they are the size a healthy one year old fish should be.