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
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Whatever specific topic brought you here, four fundamentals govern long-term aquarium success: water quality, parameter stability, biological filtration, and species-appropriate husbandry. Skip any one and the others struggle to compensate.
Water quality: ammonia + nitrite at zero, nitrate under 30 ppm freshwater + 10 ppm reef. Test weekly with API or Salifert kits. Use our water parameter checker to score your readings against your tank type.
Parameter stability: stable wrong parameters beat fluctuating ideal parameters. Most fish tolerate a wide pH range if it's stable. Sudden swings of 0.4+ pH or 5+°F kill fish faster than chronic suboptimal values. Use temperature controllers (Inkbird) + automated dosing for consistency.
Biological filtration: the bacterial colony on your filter media + rock + substrate is the engine. Never replace all media at once. Use our filter turnover calculator to size correctly.
Species-appropriate husbandry: research adult size, territoriality, diet, and tankmate compatibility before purchase. Use our tank stocking calculator + compatibility guides.
How long does an aquarium take to set up? 4-6 weeks for full cycling + first stocking. Use our cycle ETA calculator + how long does cycling take.
What's the best aquarium for beginners? 20-gallon long. Big enough for parameter stability, small enough for budget + space. See beginner picks.
How often should I do water changes? 25-30% weekly. See water change frequency Q&A + water change calculator.
Why does my fish keep dying? 5 leading causes: uncycled tank, wrong species pairings, no quarantine, undersized tank, neglected water-change schedule. See full diagnosis.
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