“Why is my TDS not zero?” — Stage 6 is supposed to raise TDS. That’s the feature working correctly.
Standard RO water (post-Stage 4) reads 10–30 mg/L TDS — nearly pure. Stage 6 (the FA15 alkaline filter) intentionally adds dissolved calcium and magnesium back into that water, raising TDS to 30–80 mg/L. A TDS meter cannot distinguish between healthy minerals and contaminants. To verify the RO membrane is working, measure TDS at Stage 4 output (between Stage 5 and the FA15 inline filter) — 90%+ reduction from your feed water should be clearly visible there. The TDS increase you see in the final output is the alkaline filter performing as designed.
System Specifications
| Model | RCC7AK — 6-stage with alkaline remineralization |
| Certification | NSF/ANSI 58 — WQA Gold Seal (whole system) |
| Daily production | 75 GPD (at standard test conditions) |
| Feed water pressure | 40–80 PSI (optimal: 60–80; booster pump needed below 45 PSI) |
| TDS rejection (Stage 4) | 90%+ NSF certified; 96–98% in controlled testing |
| Output pH (after Stage 6) | 8.5–10+ (independent lab: 9.1 confirmed) |
| Storage tank | 3.2-gallon pressurized (pre-pressure: 7–10 PSI empty) |
| Wastewater ratio | ~3:1 drain-to-product (varies with feed pressure) |
| Annual filter cost | ~$65–$95/year (~$0.17/gallon) |
| Warranty | 1-year limited • Lifetime technical support (Atlanta, GA) |
| Retail price | ~$199–$280 (Amazon/Home Depot 2024–2026) |
All Six Filter Stages
| Stage | Filter | Part # | Replace | Key function |
|---|---|---|---|---|
| 1 | PP Sediment (5 micron) | FP15 | 6–12 months | Sand, silt, rust, sediment. Transparent housing for visual inspection. |
| 2 | GAC Carbon Filter | FG15 | 6–12 months | Chlorine, VOCs, chloramines, taste/odor, mercury, lead. Rubber washer faces UP. |
| 3 | CTO Carbon Block (5 micron) | FC15 | 6–12 months | Deep chlorine/chloramine removal; final protection for RO membrane. |
| 4 | 75 GPD RO Membrane (TFC) | MC7 | 2–3 years | Lead, arsenic, fluoride, nitrate, TDS, cysts, PFAS. Insert small O-ring end first. |
| 5 | Post Carbon Inline (T33) | FT15 | 12 months | Final taste/odor polish of stored tank water before faucet. |
| 6 ★ AK | Alkaline Remineralization Filter | FA15 | 12 months | Adds Ca²+ & Mg²+; raises pH to 8.5–10+; produces alkaline mineral water. |
How Stage 6 Alkaline Remineralization Works
Stage 6 is what distinguishes the RCC7AK from every other iSpring model. After Stage 4 removes 90–98% of dissolved solids (producing near-pure water with a pH around 7.0), Stage 6 re-introduces select minerals through a controlled dissolution process.
flat taste for some users
dissolves Ca²+ & Mg²+
raises alkalinity
carbonate +17,775%
Source: Water Filter Guru independent lab testing (Brian Campbell, WQA CWS), March 2026. pH increase from neutral 7.0 to 9.1 is comparable to premium alkaline bottled water brands.
The FA15 filter contains a blend of calcite (calcium carbonate) and Corosex (magnesium oxide). As near-pure RO water passes through, it dissolves a small amount of this mineral media — releasing calcium and bicarbonate (HCO₃²) ions. The dissolution rate is self-regulating: as the water’s mineral content rises and pH increases, dissolution slows, preventing over-mineralization. This is the same process that gives spring water its characteristic mineral taste.
Independent Lab Performance Data
Water Filter Guru (Brian Campbell, WQA Certified Water Specialist) published independent lab testing of the RCC7AK in March 2026, including SimpleLab Tap Score analysis. Key findings:
RCC7 vs RCC7AK — Which One?
| Feature | RCC7AK | RCC7 |
|---|---|---|
| Price | ~$199–$280 | ~$170–$270 |
| Stages | 6 stages | 5 stages |
| Output pH | 8.5–10+ (alkaline) | ~6.5–7.0 (neutral RO) |
| Added minerals | Yes — calcium & magnesium bicarbonate | No |
| Stages 1–5 | Identical to RCC7 | Identical to RCC7AK |
| Annual filter cost | ~$65–$95 | ~$50–$75 |
| Certification | NSF/ANSI 58 (WQA Gold Seal) | NSF/ANSI 58 (WQA Gold Seal) |
| Best for | Buyers who want mineral-balanced alkaline water, spring water taste | Buyers who want maximum purity; prefer to add minerals via separate filter later if desired |
Installation Overview
The RCC7AK installation is identical to the RCC7 through five stages. The only additional step is connecting the FA15 alkaline filter inline between Stage 5’s blue output tubing and the RO faucet. The FA15 comes with quick-connect elbow fittings and mounting clamps.
For the complete step-by-step installation walkthrough — including tank pre-charge pressure check, drain saddle placement, membrane installation, and the 10 most common installation mistakes — see our dedicated guide: iSpring RCC7AK Installation Guide (Step-by-Step, No Mistakes) →