Maurice: They are indeed beautiful, but far too rampant in a subtropical climate. As your fry grow, don't be surprised if the roots get eaten. My experience has been that when the koi reach about 15", it becomes a challenge to keep the hyacinth healthy. The roots should be 10-12" long when not being eaten faster than they can grow. A nice living food for your young ones. Note the veining in the hyacinth leaves? Should be solid deep green in shade, or solid bright medium green in sun. The veining indicates growth is outpacing nutrient levels ... just what you are seeking. When frost singes the leaves, it is time to pull them out & add to the compost pile. Otherwise, an awful rotting mess on the pond bottom. BTW, you can winter over a few easily indoors as long as they get some sun or a lot of artificial light. Also, can over winter out of water by laying on top of damp sand in a plastic container, cover with plastic and place where some sun will reach them . ... easier to spend a little each year to replace, but some enjoy the DIY experience.
My first experience in "business" was raising water hyacinth for an aquarium shop when I was about 13. The shop acquired several for me in April & I'd place them in ponds made from sunken kiddie pools. By late May I could deliver a dozen a week to the shop for the entire summer. I received a store credit of 10 cents per plant, which retailed for 49 cents. Bought a lot of 39 cent tropicals that way. ....At current exchange rates, you paid about $4 per plant. There are a few billion in the drainage ditches along the Florida Turnpike. They're free to all who dare take the risk of gators and snakes.
Khanh: No soil. Will do very well if roots touch muddy/mulm bottom, but roots rot if buried. Need to float freely in water column. Hyacinth can be sensitive to pond treatments/chemicals. Salt can be deadly.