When You Need RO
Discus, soft-water rams, cardinal tetras — need TDS under 100ppm.
Tap water with high TDS (300+ppm) — too hard for sensitive species.
Reef tanks always need RO/DI water for accurate salt mixing.
Equipment
4-stage RO unit: $80-200 for serious aquarist setup.
TDS meter — verify output (should be under 10 ppm).
Storage container (5-gallon bucket).
Adding Minerals Back
Equilibrium (Seachem) or Salty Shrimp Discus — adds GH and trace minerals.
Don't use marine salts in freshwater — wrong mineral composition.
Cost vs Benefit
Initial: $100-300. Ongoing: ~$0.10 per gallon (membrane lifespan, prefilter replacement).
Worth it for sensitive species. Overkill for hardy community tanks.