Add ag lime or gypsum. From the fact sheet

Hardness
can be a mixture of divalent salts;
however, calcium and magnesium
are the most common sources of
water hardness.

Calcium and magnesium are essential
in the biological processes
of fish (bone and scale formation,
blood clotting and other metabolic
reactions). Fish can absorb calcium
and magnesium directly
from the water or from food.
However, calcium is the most important
environmental, divalent
salt in fish culture water. The presence
of free (ionic), calcium in culture
water helps reduce the loss of
other salts (e.g., sodium and potassium)
from fish body fluids (i.e.,
blood).

Where hardness is
caused by limestone, the CaCO3
value usually reflects a mixture of
free calcium and magnesium with
calcium being the predominant divalent
salt.
Agricultural limestone can be used
to increase calcium concentrations
(and carbonate-bicarbonate alkalinity)
in areas with acid waters or
soils. However, at a pH of 8.3 or
greater, agricultural limestone will
not dissolve. Agricultural gypsum
(calcium sulfate) or food grade calcium
chloride could be used to
raise calcium levels in soft, alkaline
waters. Expense might be prohibitive
if large volumes of water
need treatment.