Martin, have you ever had this problem? Ive never lost any fish when it dies in my ponds.
No, but like you I removed it. I only used hyacinth to keep the pond from getting choked with algae while it was new. Now it's overgrown with Salvinia, but I've put in some pickerel weed and some other stuff I've found in my travels.
Water hyacinth is not toxic. Garden hyacinth (the bulb), however, is very toxic when eaten, and handling the bulbs can give a nice skin rash (don't ask me how I know this).
Fish kills associated with water hyacinth are, however, very commonly documented. Since there is so much biomass, when the stuff dies in the fall it quickly uses up the oxygen in the water as it decomposes. Not only can the fish die of a lack of oxygen, but toxic chemicals can be released from the rotting organic matter in the now anoxic water (e.g. sulfur dioxide).
Interestingly, water hyacinth has also been blamed for starving people in a couple of developing contries by blocking the waterways used for travel/fishing/etc.