Total dissolved solids, salinity, and conductivity in marine, freshwater, planted, and shrimp tanks.

TDS stands for total dissolved solids — a measure of all the minerals, salts, and organics dissolved in the water. It’s reported in ppm and is a quick proxy for overall water quality and consistency between water changes.

• → Read: TDS explained for aquariums

Most freshwater fish are comfortable across a wide TDS range, typically 200–500 ppm. The exact value matters less than stability. Rising TDS between water changes signals build-up of waste and minerals.

Caridina shrimp typically want 100–160 ppm; Neocaridina prefer 200–300 ppm. Continuous monitoring is far more useful than weekly hand-testing because shrimp are sensitive to fast TDS changes.

Most non-shrimp planted tanks run 150–350 ppm. Higher TDS is fine if it comes from fertilizers, but if it’s climbing because of waste, your water-change schedule is too light.

Rising TDS between water changes is normal — you’re adding food, ferts, and the fish are producing waste. Faster-than-usual rises can mean uneaten food, a dead fish, or evaporation concentrating minerals. Logging TDS makes the pattern obvious.

Minerals from food, fertilizers, top-off water, and broken-down waste accumulate over time. Water changes dilute them. A continuous TDS reading shows you exactly when the next change is due.

Salinity is the concentration of dissolved salts in the water. Marine fish, inverts, and corals are adapted to a narrow band; drift kills sensitive species. Continuous salinity monitoring is the closest thing reef-keepers have to a safety net.

Most reef keepers target 1.025 specific gravity (about 35 ppt). Fish-only tanks tolerate slightly lower (1.020–1.022). Stability matters more than the exact target, especially for SPS corals and inverts.

• → See how Tank Commander helps reef tanks
• → Automating a marine tank — guide

Salinity (ppt) is a direct measure of dissolved salts; specific gravity (sg) compares the density of saltwater to fresh water. They’re related but not identical, and salinity is generally the more accurate target.

A refractometer is a manual optical tool — accurate but needs calibration and can only give you a single reading. A conductivity probe streams a continuous salinity value to a controller, which is dramatically safer for reef tanks.

Almost always evaporation (rising salinity) or a malfunctioning auto top-off (falling salinity from overshooting). Continuous monitoring catches drift hours before livestock react.

• → Continuous monitoring vs spot testing
• → Salinity monitoring with Tank Commander

An ATO replaces evaporated fresh water, holding salinity stable. If the ATO sticks on or overshoots, salinity falls fast. Tying the ATO outlet to a salinity threshold is a strong second line of defense.

• → Automating a marine tank — full guide
• → For saltwater fish-only

Yes — uncovered tanks in dry climates can evaporate enough water in a day to noticeably raise salinity. This is why reef keepers run automatic top-offs and watch salinity continuously.

Replace small amounts of tank water with RO/DI water over hours, not minutes. Aim for a salinity change of no more than 0.001 sg per hour for sensitive corals and inverts. Continuous monitoring lets you see the slope live.

Pre-mix a higher-salinity batch with quality salt mix and dose it in slowly over hours. Avoid pouring dry salt into the display — it creates damaging local hot spots. Watch the live salinity reading until you’ve hit your target.

RO/DI water should read at or near 0 ppm. A rising TDS from your RO/DI output means your DI resin is exhausted and minerals are passing through. Catching this early protects sensitive tanks from chloramine, copper, or silicate ingress.

Sediment and carbon stages typically last 6 months; membranes last 1–2 years; DI resin should be replaced when post-filter TDS climbs above 0–1 ppm. A continuous TDS reading takes the guesswork out.

Plants need minerals, but extremely high TDS (often from old water) signals build-up of waste rather than useful nutrients. If TDS climbs steadily despite ferts, your water-change frequency is too low.

Yes. Sudden TDS swings interfere with shell formation and can kill shrimp during a molt. Stability is more important than exact value, which is why continuous monitoring is so valuable for shrimp keepers.

A conductivity probe in your sump or display streams a live salinity reading. You can view it on the device, in the app, and set alerts for any drift outside your safe range.

Yes. Set a high and low salinity threshold and Tank Commander pushes a notification and sounds a local alarm the moment the reading crosses either. You can also tie an outlet (like the ATO) to the same value.

Yes. For example, a top-off pump can be set to run only while salinity is above a target, and stop the moment it drops to the safe value. This prevents the classic ‘ATO got stuck on’ disaster.

Yes — saltwater is dramatically more conductive, which is why salinity probes use conductivity to measure salt content. It’s also why stray voltage in marine tanks demands more attention.

Yes. Tank Commander has profiles for marine and freshwater so the appropriate parameters and ranges are shown for your tank type. The same hardware works for both.

No — TDS is a useful overall water-quality indicator but it doesn’t tell you which dissolved substance is changing. You still want pH (continuously) and periodic KH tests, especially in reef tanks.