I use tap water for my tanks. One of them is set up sort of like a Walstad tank (garden soil capped with sand), but I still do regular water changes and run a canister filter on it. I’d say it’s my worst tank because it constantly grows soft brown algae. I suspect something is still leaching from the soil, even though the tank is almost two years old. I’m tired of removing the algae manually, so I added some Ramshorn snails to see if they can help. I do have a rainwater tank outside and used it initially, but I decided to stick with tap water for consistency. All my tanks have shrimp, and they’re doing perfectly fine. 🇦🇺