SLF Water Filtration Comparison · 2026
VIVA21 6-Stage Alkaline RO
Recommended
RKIN Flash RO
Good fit for shoppers prioritizing a compact traditional under-sink RO system with optional remineralization and no electricity requirement.
VIVA21 6-Stage Alkaline RO
Choose the VIVA21 6-Stage if you want an affordable traditional under-sink RO system with alkaline remineralization and stronger certification positioning than many budget RO systems.
RO Performance & Ownership
Compare RO output, filtration stages, remineralization, contaminant evidence, certifications and ownership cost.
Which Is Right for You?
VIVA21 6-Stage Alkaline RO
Best for: Households wanting a traditional under-sink RO system with remineralization and a dedicated storage tank.
Consider if: Budget Under $300, Want Remineralization
RKIN Flash RO
Best for: Households wanting a compact under-sink reverse-osmosis system with strong filtration and the option to remineralize the purified water.
Consider if: Want Remineralization
Our Recommendation
VIVA21 6-Stage Alkaline RO
A compelling value-oriented alternative to higher-priced tankless RO systems for households that don't mind giving up cabinet space for a conventional storage tank.
RKIN Flash RO
A flexible traditional under-sink RO option, especially for buyers who value non-electric operation and selectable post-filtration.
The Short Answer
Choose the VIVA21 6-Stage if you want an affordable traditional under-sink RO system with alkaline remineralization and stronger certification positioning than many budget RO systems.
Choose the AlcaPure version for calcium/magnesium remineralization and alkaline 7-8 pH; choose OnliPure if zero-TDS water is the priority.
Water Filter Comparison FAQs
Answers are generated from the current SLF product records for these two systems.
Which is better, VIVA21 6-Stage Alkaline RO or RKIN Flash RO?
VIVA21 6-Stage Alkaline RO is the overall pick for this specific matchup. Choose the VIVA21 6-Stage if you want an affordable traditional under-sink RO system with alkaline remineralization and stronger certification positioning than many budget RO systems.
Which system has the higher daily filtration capacity?
RKIN Flash RO has the higher rated output: 75 GPD versus 50 GPD.
Which system offers remineralization?
Both systems are listed as yes for remineralization in the current SLF records.
Which system has the lower MSRP?
VIVA21 6-Stage Alkaline RO has the lower MSRP in the current SLF records at $339.99, compared with $499.95 for the other system.
Do both systems require a plumbing connection?
VIVA21 6-Stage Alkaline RO: Yes. RKIN Flash RO: Yes.
Which system is more renter friendly?
VIVA21 6-Stage Alkaline RO: No. RKIN Flash RO: No. Check lease and plumbing-modification rules before installing an under-sink system.
What certifications are listed for these systems?
VIVA21 6-Stage Alkaline RO: Manufacturer states VIVA21 6-Stage Alkaline RO (V21RS6S01PH) is WQA Gold Seal certified to NSF/ANSI 58 for RO system performance and NSF/ANSI 372 for lead-free materials. WQA Gold Seal is third-party product certification. SLF has not independently confirmed the exact model in WQA's public certified-product directory; retain manufacturer-source qualification pending direct directory verification. RKIN Flash RO: RKIN states that the 75 GPD reverse osmosis membrane used in the Flash system is NSF/ANSI 58 certified. This is component-level evidence for the RO membrane; exact-model NSF/ANSI 58 certification of the complete Flash RO system was not confirmed.
How do estimated annual filter costs compare?
VIVA21 6-Stage Alkaline RO: $60.00. RKIN Flash RO: $145.44. Actual replacement timing and prices can vary.