water softeners put sodium (Na+) ions into the water. and it can be significantly
high enough to cause real problems with GF, so I imagine it can with koi.

The relationship between salt (NaCl) and high blood pressure is complex and just
lowering sodium chloride intake may have no effect at all, or actually worsen blood
pressure. The suggestion is that the balance of total salts is important and
unbalanced (high sodium to potassium, etc.) intake is what can cause problems.

It is the chloride ion that tastes "salty". To increase the saltiness of foods, add
lemon or vinegar. To decrease the saltiness add a pinch of baking soda (which is how
to cut the acid in tomato sauces as well). Ingrid

I think that may be your problem. A water softer that uses salt puts enough
sodium in the water that those with high blood pressure are told not to
drink the water from such.