Basically all a water softener does is remove hard minerals from the water. Mostly Calcium and Magnesium . Unless the water in your area is really hard it's not even going to be that much. If the lizards drink it that's all matters. You'll never know how different your is water is from the water they drink in the wild but...........
Water is water . In the dry months my animals get Ultra hard tap water in the rainy season they drink fresh rain water . No difference. Monitors only drink to hydrate .
For fish it's more complicated then that . Fish live in water and process it thru their gills. Water quality affects their breathing ability and has direct physical relation to their skin, gills and eyes. Different species have adapted to different water conditions. For fish water is a lot more then a drink.
Ernie Eison
WESTWOOD ACRES REPTILE FARM INC.