Never fill from a garden hose

That number drives your tank sizing more than anything else, and most people guess it low.

Sourcing, honestly:

Municipal fill — easiest. Use a dedicated potable-water hose, never a garden hose. Standard hoses leach chemicals and grow biofilm inside. A drinking-water hose is cheap and fixes a problem most people never realize they have.

Rainwater — genuinely excellent input water; it starts nearly distilled. Divert and discard the first flush (that's your roof dirt), then filter and treat. Check local rules — catchment law varies by state.

Surface water — always treat. Look upstream: livestock, campsites or farmland means viruses in play, which means purification, not just filtration.

Wells and springs — can be excellent or contaminated, and you cannot tell by looking. A lab test is cheap and worth it before you rely on a source.

Storage rules: food-grade containers only (HDPE #2). Cool and dark — light plus warmth grows algae. Sanitize tanks with dilute unscented bleach twice a year. Keep an emergency reserve separate from the daily tank.