Read this and see what fits your situation.
Both calcitic limestone (CaCO3)
or dolomitic limestone
[CaMg(CO3)2] are good.
Calcium carbonate hardness is a
general term that indicates the
total quantity of divalent salts present
and does not specifically identify
whether calcium, magnesium and/or some other divalent salt is
causing water hardness.
https://srac.tamu.edu/index.cfm/event/getFactSheet/whichfactsheet/112/ 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). Sodium and potassium
are the most important salts in fish
blood and are critical for normal
heart, nerve and muscle function.
.gif)
