Well it should be O.K. A water softener just exchanges ions in your water. They take out the Calcium and Magnesium and put it Potassium and Sodium. Water softener really don't do much in terms of water and how it affects your fish but might be important in plant growth. Some plants prefer softer water to grow. And realize the more Calcium and Mg you take out, your KH or hardness levels do go down and might influence your pH fluctutations if you don't monitor it closely.