??? Something strange happening. Water hyacinth do fine around here in ponds with hardness of 350ppm and pH of 8.5.
Is the pond just 600 gals.? If so, a comparatively low cost RO unit could be used. The amount of waste water would not be too great & can be used for other purposes. Got a dog to wash?
You may want to wait until after you get the chemical analysis. The Ag analysis will be better suited for your purpose than drinking water analysis. For drinking water the levels of various compounds acceptable for human consumption are quite higher than what is appropriate for koi, especially among the metals. Pay particular attention to copper & zinc levels. Water hyacinth don't like either one in excess... and not good for koi. The hyacinth also dislike salt.