A water softener may seem extreme but I recommend it. Less water spots, including for washing car, and feels better (opinion). As far as the fish go... My filters last 12 months resin lasts about 3 months, and I get 3 tds out of a 2x75 gpd ro setup. I also get damn close to 1:1 water output. This is for a 300+ gal setup, drinking water, and ice.
Dual di canisters rotating 2nd phase to first phase when replacing resin seems to help quite a bit with resin life too.