Fish only. Honestly in my experience I’ve kept & still have fish only tanks with straight tap water without treating it & I live in an old building built in the 1910s so plumbing is old. To this day no issues but I do keep a pretty dim lighting setup, cheap LEDs to avoid excess algae growth. Tank runs on a fluval fx5, the big boy & I do keep a good amount of carbon & tons of bio media in the canister. Kept it like this for over 10 years & no issues. Yes it’s saltwater, sand & lace rock I bought at a quarry for $0.35/lb. It’s how I was thought to build them when I was a kid in the 90s. Again, this is my own experience & things can differ for others but it has worked for me. Not recommending my personal approach but straight tap has work for me. Good luck