An RO/DI system is non-negotiable for reef tanks and Caridina shrimp tanks. Tap water contains chlorine, chloramines, copper, phosphate, silicate, and trace heavy metals - all of which feed nuisance algae or kill sensitive livestock. A properly tuned RO/DI system produces 0 TDS water at 75-150 gallons per day.
Reef tank: top-off (1-3 gal/day) + water changes (10-20% weekly) = ~25-50 gallons/week for a 100-gallon system. Sized at 75 GPD, that's under 1 hour of run time per week. Sized at 150 GPD, run time halves but waste water also doubles.
Standard 4-stage: sediment + carbon + RO membrane + DI resin. Bulk Reef Supply 6-stage: adds a second carbon stage (better for chloramines) and a second DI stage (longer resin life). 4-stage is fine for most municipal water. 6-stage for chloraminated water (most US cities).
Bulk Reef Supply (BRS) - the reef hobby standard, $150-350, parts always in stock. SpectraPure - higher-end, longer-life membranes, $300-600. AquaFX - similar to BRS, slightly cheaper. Avoid: generic Amazon brands - membrane fouls in 6 months.
Buy a $15 handheld TDS meter. Test tap water (typical 100-400 TDS), RO output (should be 5-15 TDS), and DI output (must be 0 TDS). DI resin exhausts when output rises above 0 TDS - replace immediately.
Sediment + carbon: every 6 months. RO membrane: every 2-3 years (or when TDS rejection drops below 95%). DI resin: when output exceeds 0 TDS (typically 6-18 months depending on source water). Color-changing DI resin (green to amber) signals exhaustion visually.
RO membranes need 50-65 PSI to produce rated GPD. If your house water pressure is below 50 PSI, add an Aquatec booster pump ($150-200). Improves production rate, extends membrane life, and reduces waste-water ratio.