I have not seen much difference in water sprite growth due to water hardness or softness. I have seen iron deficiency---pale new growth---and potassium deficiency---holes in older leaves, older leaves disintegrating. Both deficiencies are easily corrected by additions of soluble iron or potassium. Water sprite does not require high light. It is easiest to maintain in moderate light tank, (around 2-3 watts fluorescent per gallon) I have gotten it established in a 15 gallon tank with only one 20 watt T12 fluorescent tube.