All plants thrive when given fertilizer.  Pond plants aren't any different than our garden plants and flowers.  A little tip for your water hyacinths is to take them out of your pond and place them in a tub with water and miracle grow in it.  Leave them soak for awhile soaking up the fertilizing in their leaves.   Rinse them off and return them to your pond.  They will flower more for you.   You can also do this when you first receive your plants when re-hydrating them prior to planting them or placing them in your pond.