TDS explained: what total dissolved solids mean for your tank

Total Dissolved Solids — TDS — is a single number that tells you how 'mineralized' your aquarium water is. It's not as specific as a full water test, but it's enormously useful as a continuous indicator of water quality, water-change cadence, and the health of your RO/DI system. For shrimp keepers especially, it's arguably the single most important parameter to track.

What TDS actually measures

A TDS meter sends a small electrical current through the water and measures conductivity. Pure H₂O conducts almost nothing; dissolved minerals and salts conduct readily. The meter converts the conductivity reading into an estimated TDS value in parts per million (ppm).

TDS doesn't tell you what's dissolved — only how much. Calcium, magnesium, sodium, chloride, fertilizer salts, and broken-down waste all contribute. Two tanks with identical TDS readings can have very different chemistry. That's a limitation, but it's also why TDS is so useful as a 'something changed' alarm rather than a diagnostic test.

What good TDS values look like

• Pure RO/DI output: 0–1 ppm. Anything higher means your DI resin is exhausted.
• Tap water: highly variable, typically 100–600 ppm. Worth knowing your local baseline.
• Caridina shrimp tanks (Crystals, Tigers): 100–160 ppm.
• Neocaridina shrimp tanks (Cherry, Yellow): 200–300 ppm.
• Most freshwater community tanks: 200–500 ppm.
• Heavily planted, fert-dosed tanks: 300–500 ppm depending on regimen.
• Reef tank: don't use TDS — use salinity (around 35 ppt or 1.025 sg).

Why rising TDS matters

TDS rises naturally between water changes. Food, fertilizers, and decomposing waste add minerals; evaporation concentrates whatever's already dissolved. A graph of TDS over a week looks like a slow ramp. What matters is the slope and the ceiling.

• Faster-than-usual ramp: too much food, too many fish, or a dead body somewhere in the tank.
• Higher ceiling than last week: top-off water is more mineralized than usual (your RO is going).
• Sudden jump: medication, fertilizer overdose, or accidental tap water during top-off.

Without continuous monitoring, all three patterns are invisible. A weekly hand-test catches the average and misses the slope.

Why TDS matters most for shrimp

Shrimp molt by absorbing water through their carapace as they slip out of the old shell, then quickly hardening the new shell with minerals from the surrounding water. This process is exquisitely sensitive to TDS — a swing of even 50 ppm during a molt can be fatal.

Continuous TDS monitoring is the single biggest improvement most shrimp keepers can make. Sudden changes during top-offs, water changes, and remineralization are easy to spot and correct before any shrimp are lost.

Using TDS to schedule water changes

Most aquarists do water changes on a calendar — weekly or bi-weekly, whether the tank needs it or not. A more honest signal is TDS. Set a target ceiling (say, 150 ppm above your fresh-water TDS value), and change water when you hit it. Lightly stocked tanks need fewer changes; heavily stocked tanks need more. The TDS chart tells you the truth.

Watching your RO/DI output

Place a TDS meter (or a continuous probe) on the output of your RO/DI unit and you'll get a free preventive-maintenance system. Sediment and carbon stages last about 6 months; membranes last 1–2 years; DI resin needs replacement when post-filter TDS climbs above 0–1 ppm. Without continuous monitoring you're guessing.

Bottom line

TDS isn't a substitute for ammonia, nitrite, nitrate, or alkalinity tests — but as an always-on indicator of overall water quality and a guard against silent problems, it's hard to beat. For shrimp keepers, it's not optional. For everyone else, it's the easiest single number to add to your monitoring stack.