TDS & RO/DI: When (and When Not) to Use Purified Water
1) TDS Demystified: What It Measures and Why It Matters
TDS (Total Dissolved Solids) is a catch‑all number for the dissolved ions in water—calcium, magnesium, potassium, sodium, bicarbonate, chloride, sulfates, nitrates, traces of metals, etc. Handheld pens measure electrical conductivity (EC) and convert it to parts per million (ppm) using a factor; they’re quick and affordable, but they don’t tell you which ions are present—only the sum. For aquarists, that’s both a limitation and a superpower: while TDS isn’t a substitute for pH, GH, and KH testing, it’s a fantastic trend metric to track overall mineral load, fertilizer accumulation, or whether your top‑off strategy is drifting your water too hard over time.
How to interpret common ranges. Most municipal tap waters fall between ~50–500 ppm TDS depending on geology and treatment. Soft, peat‑influenced sources may be 50–100 ppm; limestone‑rich regions can exceed 300 ppm. For general community tanks (tetras, rasboras, livebearers, corydoras, hardy plants) a mid‑range TDS—say 120–220 ppm—often ensures enough hardness for osmoregulation and plant metabolism without stressing soft‑water species. African rift cichlids thrive in higher mineral content; blackwater specialists prefer significantly lower dissolved solids paired with low KH.
What TDS is not. It’s easy to assume “higher TDS = bad,” but TDS doesn’t measure toxins. A clean solution of calcium and bicarbonate reads high TDS but is harmless; a low‑TDS sample could still contain ammonia or chloramine. That’s why the TDS pen lives next to your liquid tests, not instead of them. Use TDS to understand dilution and drift: after dosing fertilizers in a planted tank, TDS jumps; after a water change, it drops; after top‑off with hard tap water, it creeps up—because evaporation removes water, not minerals.
Practical use day‑to‑day. Record your baseline TDS right after a water change. Through the week, watch it climb as minerals concentrate and fish waste is processed into nitrate. When your TDS is rising faster than usual, that’s a clue to review feeding, maintenance, or filter performance. If you run shrimp tanks, TDS becomes a convenient guardrail for remineralization. Many Neocaridina keepers target ~170–220 ppm; Caridina variants may prefer ~110–150 ppm with specific GH/KH balances. Remember, TDS is only a hint—always pair with GH/KH and pH tests to confirm the ionic type.
Measurement quirks. Pens use different conversion factors (0.5 vs 0.7). Two pens can disagree by 40%. The absolute number matters less than consistency. Pick one device, log trends, and calibrate occasionally using a standard solution. Temperature compensation is built into most modern pens, but don’t obsess over the last digit—±10 ppm is within normal variance.
Takeaway. TDS is your dashboard’s fuel gauge. It won’t diagnose the engine, but it tells you when something is trending off. Use it to sanity‑check remineralization, to choose water change percent, and to validate that your buffer strategy is stable from week to week.
2) RO/DI Basics: How Systems Work, Remineralization, and Targets
RO/DI stands for Reverse Osmosis plus De‑Ionization. An RO membrane forces water through a microscopic barrier that rejects most ions and organics; DI resin then “polishes” what slips through, exchanging remaining ions for hydrogen and hydroxide to yield nearly pure H₂O. Typical output after good RO/DI is 0–5 ppm TDS. That water is too “empty” for long‑term fish or plant health, so aquarists remineralize it to controlled GH/KH targets using salts or commercial mixes.
Stages explained. A common 4‑stage unit uses a sediment cartridge (protects carbon), a carbon block (removes chlorine/chloramine that would damage the membrane), the RO membrane (the workhorse), and a DI canister (final polish). Some add extra carbon, a second DI, or a booster pump for low‑pressure homes. If your city uses chloramine, pick a chloramine‑rated carbon block and keep up with changes; exhausted carbon lets oxidants attack the membrane, shortening life and reducing rejection.
Remineralization: hitting targets with intent. The art is not “zero TDS,” it’s predictability. For planted tanks, remineralize to a GH that supplies calcium and magnesium (e.g., 3–6 dGH) and a KH that gives you a stable pH (e.g., 2–4 dKH for CO₂‑injected systems, 3–6 dKH for non‑CO₂). Many keepers use a single all‑in‑one remineralizer for shrimp (GH‑only for Caridina with near‑zero KH; GH+KH blends for Neocaridina). Weigh salts on a scale, mix in a food‑safe container, and confirm with GH/KH tests. TDS is a quick proxy once you’ve established your recipe: if 200 ppm equals GH 6 and KH 3 in your system, you can reproduce that number weekly with confidence.
RO vs RO/DI. If you’re keeping hardy community fish and plants, plain RO (without DI) that yields ~5–20 ppm may be sufficient once remineralized. DI is invaluable for sensitive shrimp, breeding projects, or when you need virtually ion‑free water to blend with very hard tap. DI resin costs rise with high CO₂ and silica; if you’re burning through resin fast, confirm your membrane rejection and carbon health.
Throughput and storage. A “75 GPD” label assumes optimal pressure and temperature; real output can be half that. Plan for a storage bin with a float valve and lid. Add an inexpensive TDS meter inline (one probe after membrane, one after DI) so you see when either stage is drifting. Keep tubing neat and secure—floods come from forgotten taps, not the unit itself.
Takeaway. RO/DI is a control tool. It gives you repeatable water so husbandry is about fish and plants—not your municipal geology. The goal isn’t the lowest TDS number; it’s dialing in the right minerals for the organisms you keep and the equipment you run (filters, CO₂, fertilizers).
3) When to Use RO/DI (and When to Avoid It)
When RO/DI shines. Use purified water if your tap is extremely hard/alkaline and you want to keep soft‑water species (wild bettas, many tetras, Apistogramma, Caridina shrimp), if your tap swings seasonally, if you fight stubborn algae tied to silica/phosphate in source water, or if your planted show tank needs consistent carbonate for precise CO₂ pH‑drop control. RO/DI is also ideal for shrimp‑only nanos where small errors accumulate fast, and for breeders aiming to replicate blackwater or rainwater conditions.
Blend, don’t overcomplicate. Many aquarists succeed by blending a percentage of RO/DI with tap to hit a target KH/GH without handling salts—e.g., 70% RO/DI + 30% tap to land near KH 2–3 for CO₂‑injected scapes. Measure your tap’s GH/KH once a month; municipalities can adjust wells or treatment, shifting your blend. Keep a simple chart on your reservoir: for each tank, list “X liters RO/DI + Y liters tap = target.”
When not to use RO/DI. If your tap is already moderate (e.g., GH 5–8, KH 3–5, TDS ~120–220 ppm) and your livestock are thriving, there’s no obligation to add complexity. Switching to RO/DI without a plan can crash pH/buffering and destabilize plants. Livebearers (guppies, mollies, platies) and many snails prefer mineral‑rich water; stripping minerals and failing to remineralize invites shell erosion and poor fry development. For goldfish or African cichlids, RO/DI often creates unnecessary work—focus on good filtration and water changes instead.
Acclimation and consistency. Whether you blend or remineralize from zero, keep consistency king. Match TDS (±20 ppm) and temperature during water changes to avoid osmotic shock. Log your recipe, dose the same way every time, and re‑check TDS before pumping water into the tank. If you change fertilizers or stocking density, expect your weekly TDS drift to change—update your water‑change percentage accordingly.
Special notes for blackwater and botanicals. Tannins can lower pH with minimal change to TDS because they’re weak acids. Don’t chase a TDS number here; target GH/KH that keeps pH stable and let the tea‑colored hue develop naturally. Pre‑soak leaves and cones, and remember that blackwater aquariums are about gentle chemistry and heavy biofilm—not ultra‑pure water alone.
4) Pitfalls, Maintenance & Cost: Making RO/DI Practical
Waste ratio & efficiency. RO membranes generate a waste stream to carry rejected ions to drain. Older units may run 1:3 (one part product to three parts waste); modern membranes and good pressure can approach 1:1. Colder water reduces output and efficiency. If the waste bothers you, collect it for lawn watering or laundry pre‑soak where safe.
Membrane care. Keep carbon fresh so chloramine never hits the membrane. Install a pressure gauge—low pressure means poor rejection and higher DI costs. Flush kits aren’t magic but can extend life by clearing debris. If the post‑membrane TDS (usually inline probe “IN”) starts climbing above spec, it’s time to verify pressure, temperature, and prefilters. If “OUT” after DI rises above 2–5 ppm sooner than usual, the resin is exhausted; replace or recharge.
Costs & logistics. A 4‑stage 75 GPD unit plus a 20–30 L storage bin, float valve, and tubing is affordable and reliable. Ongoing costs are sediment/carbon cartridges (3–6 months typical), DI resin (variable; depends on source water), and an occasional membrane (2–5 years). Add a small pump if your household pressure is below ~50–60 psi. Label every valve, and mount the unit neatly to avoid confusion for family members.
Safety & flooding prevention. The real risk isn’t “RO water kills fish”—it’s accidental overflows. Use float valves, timers, and never walk away during filling until you’ve tested your system for a week. Secure tubing, and add an auto‑shut‑off valve if your unit supports it. Keep electronics off the floor, and route hoses so they cannot siphon a tank dry.
Checklists that keep you sane. (1) Test tap GH/KH/TDS monthly; log results. (2) Record your remineralization recipe and target TDS for each tank. (3) Before every change, confirm TDS of the new water. (4) After changes, note tank TDS and observe livestock. (5) Maintain RO/DI prefilters on schedule and replace DI before it hits 2–5 ppm output for sensitive setups. With this routine, RO/DI becomes a quiet background process that supports healthy, stable aquariums rather than a source of anxiety.
FAQ
Is 0 ppm TDS water safe for fish?
Not by itself. Pure RO/DI water lacks essential minerals and buffering. Always remineralize or blend with tap to reach suitable GH/KH and a stable pH.
Do plants need GH and KH?
Yes. Calcium and magnesium (GH) are vital for growth and cell function. KH provides buffering that stabilizes pH, especially important in CO₂-injected tanks.
Can I just top off with RO/DI and never remineralize?
Top-off with RO/DI is fine to offset evaporation, but your weekly change water must still be remineralized or blended. Evaporation removes water, not minerals.
Next reads: Water Parameters Demystified • How to Test Your Water • Plant Substrates & Root Tabs • pH Crashes & KH Buffers • Shrimp-Only Nano
Labels: Water Chemistry, RO/DI, TDS, Advanced Tips