Tap water + dechlorinator = safe for most aquarium tanks. What's in tap water: chlorine (kills bacteria), chloramine (chlorine + ammonia, harder to neutralize), heavy metals (copper, lead in some plumbing), TDS varies by region (50-500 ppm typical). Dechlorinator basics: Seachem Prime (the standard - dechlorinates + detoxifies ammonia), API Stress Coat (with aloe vera), Tetra AquaSafe. Add per dose to all replacement water. Test your tap: pH (5.5-9 found in US tap), KH (varies wildly), TDS, chlorine level. Some regions have hard water perfect for African cichlids; others are soft + ideal for discus. When tap water is NOT safe: 1) discus + apistogramma + crystal red shrimp need RO water + remineralizer. 2) reef tanks need RO/DI (zero TDS) due to phosphate, silicate, copper in tap. 3) areas with high copper plumbing kill shrimp. RO/DI alternatives: 50-100 GPD home unit ($150-300), grocery store water exchange (~$0.40/gal but contaminated risk), bottled distilled (~$1/gal for emergencies only). Don't use: softened water (sodium replaces calcium - bad for fish) or filtered drinking water (still has TDS).
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