A TDS meter measures Total Dissolved Solids in ppm. RO/DI water should read 0-2 ppm; tap water often 100-500 ppm. Used to gauge RO/DI membrane health + remineralization.
TDS meters are the workhorse RO/DI verification tool. How they work: measure conductivity of water and convert to ppm of dissolved solids (mostly minerals + salts). Hobby readings: tap water 100-500 ppm (varies by region), out of RO membrane 5-30 ppm, out of DI resin 0-2 ppm (target). Top brands: HM Digital TDS-3 ($15-25, basic), HM Digital COM-100 (also conductivity), Apera AI-209 (lab-grade $80). Inline TDS meter: Bulk Reef Supply + Aquatic Life sell dual-probe inline meters for RO/DI systems - monitor input + post-DI output simultaneously. When to replace DI resin: when output reads 1+ ppm. When to replace membrane: when post-RO TDS rejection drops below 95% (track post-RO/input ratio). Calibration: use 342 ppm or 1413 µS/cm calibration solution every 6 months. Caveat: TDS = salt content roughly, NOT silica or organic carbon. Use ICP testing for full water analysis if running into algae/dino issues.
Reviewed by the Fast Aquatics husbandry team · Updated May 2026
A TDS meter measures Total Dissolved Solids in ppm. RO/DI water should read 0-2 ppm; tap water often 100-500 ppm. Used to gauge RO/DI membrane health + remineralization. TDS meters are the workhorse RO/DI verification tool. How they work: measure conductivity of water and convert to ppm of dissolved solids (mostly minerals + salts).
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