What exactly do you mean by filtered? Do you have a water softener in your house, or maybe you use a filter jug like Brita?
Water that is filtered through a softener is not good for fish if it is the kind that replace the calcium in the tapwater with sodium. The minerals in the softened water are totally unlike anything found in nature, and fish have not evolved to cope with water like that.
Brita water is very acidic (the calcium is replaced by hydrogen ions, which cause acidity). The cartridges also remove a lot of carbonate. I have a Brita jug and have compared freshly dawn tapwater to freshly filtered water. My pH dropped from 7.3 to below 6.0, and there was no detectable carbonate left in the filtered water.
Of course you may be using a different type of filter, if you can tell us what you are using, we'll be able to see if it's suitable.
