I grew up with farmers, worked for them and even ran a small farm for a few years. God bless them, backbone of America and the world for that matter. We'd be completely screwed without them...however...I would not take too much stock in assuming their practices validated anything. I found them very prone to superstition and rarely kept up on anything. Most of what they did came right from industry representatives selling the next hot (profitable) thing. When people from the Ag department came from the local college they were generally ridiculed as "eggheads". Farmers dumping barley into ponds is not a ringing endorsement for me. I saw them do many things for years that didn't work, though they thought did.
Water hyacinth commonly has a nitrogen fixing bacteria, Azotobacter chroococcum, which can provide nitrogen if needed. A tub of distilled water set outside, and kept topped off, will attract all kinds of life which will produce all kinds of nutrients. Dust alone will bring in a great deal of nutrients from a plant's perspective. The link between nutrients and plants in water gardening is grossly misguided. These plants need hundreds of times less nutrients to survive I think than is commonly thought. In most ponds these nutrients are available in abundance. Your pond is huge self filling compost pile.
Back many years ago I was in a water garden forum discussion about water hyacinth where some people saw great growth and in others saw no growth. In my pond they did poorly. I did some experiments. I moved some plants to a basin that had pond water flowing though it but no fish. I put some in a trashcan filled with pond water and a couple of pounds of pot ash to see if too much pot ash would hurt the plants. In both tests the plants took off while the plants in the pond continued to do badly. The healthy plants grew 3' long dense purplish root balls, big leaves and flowered. The plants in the pond had very short black roots, small unhealthy looking leaves and never seemed to grow.
The plants in the pond never died, just never took off.
The pot ash experiment was because many people thought that was the missing nutrient. But the plants in the basin with just pond water did just as well.
The best guess was that fish were eating new root growth. Some people did say they saw their fish picking at the hyacinths but most assumed they were after bugs. The reasoning goes that in some ponds the fish are more interested in the roots than in other ponds.
I don't think the experiments were conclusive, but maybe useful in setting up additional experiments.