Under-sink vs countertop: the key decision
Choose the iSpring RCC7 (under-sink) if: you can turn off the main cold water supply and install a T-valve under the sink; you have 18 x 18 inches of under-sink space for the housing and tank; and you want the dedicated chrome faucet at the sink deck permanently installed. The RCC7 installation takes 2-4 hours for a competent DIY installer. Choose the iSpring RCS5T (countertop) if: you rent and cannot install under-sink plumbing; you have under-sink space limitations; or you want a removable system that moves with you. The RCS5T setup takes 15 minutes.
RO output TDS and taste
iSpring RCC7 TDS rejection: 90-97% from source water. At 300 PPM source water (common in hard-water U.S. cities), RCC7 output is approximately 9-30 PPM TDS. Some users find very low TDS water (below 15 PPM) flat-tasting -- it has no mineral character. For these users, the RCC7AK with alkaline stage (target output: 7.5-8.5 pH with 30-80 PPM TDS from re-mineralization) produces water with a more pleasant mineral character. If TDS-related taste is not a concern, the standard RCC7 provides the most economical and straightforward system.
iSpring filter compatibility with third-party replacements
iSpring uses standard housing sizes for the pre-filter stages: 2 x 10-inch threaded cartridge housings for Stages 1-3. These accept industry-standard replacements from Pentek, Watts, and other suppliers -- not locked to iSpring-branded replacements. The Stage 4 RO membrane is a standard FILMTEC 75 GPD residential membrane (4 x 12 inches) -- compatible third-party membranes from Hydranautics, Vontron, and other suppliers are available at lower prices than iSpring-branded membranes. Stage 5 and 6 (post-carbon, alkaline) are iSpring-proprietary formats -- use iSpring branded replacements for these stages.