Municipal tap water in the US is treated for human consumption, not aquatic life. It contains chlorine, chloramine, copper from old pipes, phosphates from anti-corrosion treatments, and trace amounts of pesticides, pharmaceuticals, and heavy metals. None of these are at levels that hurt humans. All of them are at levels that hurt sensitive shrimp, soft coral, and SPS coral. Caridina shrimp in particular die in tap water within days because the dissolved-mineral profile is wildly different from their native blackwater habitat.
Reverse osmosis pushes water through a semi-permeable membrane that strips out 95-98% of dissolved solids, including most of the contaminants above. Deionization is a polishing stage with two ion-exchange resins (cation and anion) that remove the remaining 2-5%. The output is essentially pure water with a TDS reading of zero. A four-stage RO/DI unit with a sediment pre-filter, a carbon block, an RO membrane, and a DI canister is the standard setup for both reef and shrimp keeping. Six-stage units add a second carbon block and a second DI canister for redundancy on chloramine-treated water sources.
Aim for 0 ppm TDS (total dissolved solids) at the output. A reading above 2-3 ppm means the DI resin is exhausted and needs replacing. The membrane itself does not need replacing as often as people think - good membranes last 2-3 years on properly pre-filtered water. The DI resin is the consumable that runs out fastest because it carries the load when membrane efficiency drops.
Pure RO/DI water is too clean for shrimp - they need specific calcium, magnesium, and KH levels to molt successfully. Shrimp keepers add remineralizers like Salty Shrimp GH+, GH/KH+, or Bee Shrimp Mineral GH+ to bring TDS back up to the right target (usually 100-180 ppm depending on Caridina vs Neocaridina). The mineral profile from these products is matched to shrimp physiology, not to whatever your tap water contains.
BRS (Bulk Reef Supply) and AquaFX produce the most popular hobbyist units. BRS units are well-built and include excellent documentation; AquaFX runs slightly cheaper. SpectraPure is the upgrade tier - higher waste-water ratio efficiency and longer membrane life. For shrimp-only setups a small BRS 4-stage 75 GPD unit is plenty; for reef tanks running 100+ gallons of water changes a month, step up to a 100-150 GPD unit with a second carbon block.
Running the unit at low pressure (below 50 psi) kills RO membrane efficiency. So does cold incoming water - membranes are rated at 77F and produce dramatically less water at 60F. Never let DI resin sit dry between uses; it permanently loses capacity. And replace your sediment and carbon pre-filters every 6 months even if the manufacturer says 12 - cheap pre-filters extend the life of expensive membranes by orders of magnitude.