Understanding TDS (Total Dissolved Solids)

Every shrimp keeper I know has a TDS meter within arm's reach of their tank. Every fish keeper I know has never owned one. That gap is not because TDS matters for shrimp and not for fish — it matters for both. The gap is because shrimp keepers get immediate, visible feedback when TDS is wrong (dead shrimp, failed molts, no breeding) while fish keepers get slow, vague feedback (fish that "never quite thrive," shorter lifespans, recurring disease). A TDS meter is the single best $15 you can spend in this hobby after a liquid test kit. It tells you in two seconds what would take a stack of test strips and ten minutes to estimate.

This is the deep-dive companion to the water parameters overview and the broader sibling to the GH and KH deep dives. TDS overlaps with both — GH and KH are subsets of what TDS measures — but TDS also captures everything else: sodium, potassium, chloride, sulfate, trace elements, organic acids, fertilizer residues, medication residues. If you want the full picture of what is dissolved in your water, TDS is the only single number that captures it. The trade-off is that TDS cannot tell you which solids are present — only that solids are present.

What Is TDS, Exactly?

Total Dissolved Solids (TDS) is the sum of all inorganic and organic substances dissolved in a volume of water, expressed in parts per million (ppm) or milligrams per liter (mg/L — same thing). "Dissolved" means the solids are in solution at the molecular or ionic level, not suspended as particles. Sand stirred up by a fish is suspended, not dissolved — it does not count toward TDS. Calcium carbonate that has dissolved into calcium ions and carbonate ions is dissolved — it counts. The distinction matters because a TDS meter measures only dissolved solids, not suspended particles.

In practice, the dissolved solids in an aquarium are mostly inorganic ions: calcium (Ca²⁺), magnesium (Mg²⁺), sodium (Na⁺), potassium (K⁺), chloride (Cl⁻), sulfate (SO₄²⁻), carbonate (CO₃²⁻), and bicarbonate (HCO₃⁻). These eight ions account for 95%+ of TDS in most aquarium water. The remaining few percent are trace elements (iron, manganese, copper, zinc), dissolved organic compounds (tannins, humic acids), and residues from fertilizers, medications, and water conditioner. A TDS meter cannot distinguish between any of these — it just gives you the total.

The measurement is made by electrical conductivity. Pure water is a poor conductor; dissolved ions carry current between two electrodes in the meter. The meter measures conductivity in microsiemens per centimeter (μS/cm), then converts to an estimated TDS in ppm using a conversion factor (most meters use 0.5 or 0.7). This means TDS is technically an estimate, not a direct measurement — but for aquarium purposes the estimate is accurate to within 5 to 10 ppm, which is plenty for any practical use.

TDS vs GH vs KH

These three parameters overlap and confuse people. The shorthand: GH measures calcium and magnesium only. KH measures carbonate and bicarbonate only. TDS measures everything dissolved, including the calcium, magnesium, carbonate, and bicarbonate that GH and KH capture, plus sodium, potassium, chloride, sulfate, and everything else. TDS is always greater than or equal to GH + KH (in ppm), because TDS includes things GH and KH do not measure.

In typical tap water, GH in ppm is about 60 to 70% of TDS, and KH in ppm is about 30 to 50% of TDS. So if your TDS reads 200 ppm, your GH is probably around 120 to 140 ppm (7 to 8 dGH) and your KH is probably around 60 to 100 ppm (3 to 5 dKH). This is approximate and varies by water source — well water in limestone areas might have GH at 80% of TDS, while soft tap water in granitic regions might have GH at 40% of TDS. The point is that TDS gives you a quick proxy for the others.

For routine monitoring, TDS is the fastest and easiest measurement — dip, swirl, read. For diagnosing problems, GH and KH are more specific because they tell you which minerals are present. Shrimp keepers monitor TDS daily because shrimp respond to total mineral load, not just to calcium or carbonate specifically. Fish keepers can usually get by with monthly GH and KH testing and skip TDS — but if you ever keep shrimp or want to monitor fertilizer buildup in a planted tank, a TDS meter becomes essential.

Why Shrimp Keepers Care Most

Shrimp are osmoregulation machines. They constantly manage the salt and water balance in their bodies against the chemistry of the surrounding water, and they do this through their gills — which are also how they extract calcium for molting. The total dissolved solids in the water determine how hard the shrimp's osmoregulation system has to work. Too few solids (TDS below 100) and the shrimp cannot extract enough minerals for molting. Too many solids (TDS above 350) and the osmoregulation system is overworked and the shrimp becomes stressed.

Neocaridina shrimp (cherry shrimp and the various color variants — red cherry, blue velvet, yellow, orange, green, black rose, etc.) are adaptable and tolerate a wider TDS range. They thrive at 150 to 300 ppm, with 200 ppm being a common target. They will survive at 100 to 400 ppm but breeding drops off at the edges of that range. Below 100 ppm, molting failures increase. Above 400 ppm, breeding stops and lifespan shortens.

Caridina shrimp (Crystal Red, Taiwan Bee, Tiger, Bee, Bumblebee) are more demanding and need lower, more stable TDS. They thrive at 100 to 200 ppm, with 150 ppm being typical. They will survive at 80 to 250 ppm but are much less forgiving than Neocaridina — a TDS spike from a water change with the wrong tap water can kill a tank of Crystal Red shrimp in hours. This is why Caridina keepers almost always use RO water remineralized with Salty Shrimp GH+ — it lets them control TDS precisely instead of fighting whatever their tap gives them.

TDS and RO Water

Reverse osmosis (RO) water has zero TDS — the membrane strips out virtually all dissolved solids. This makes RO water the perfect starting point for shrimp keepers, soft-water fish keepers, and anyone who wants precise control over their water chemistry. You start with zero TDS, add back exactly the minerals you want with a remineralization product, and you have full control. No more fighting your tap water's quirks.

The standard remineralization products for shrimp are Salty Shrimp Bee Shrimp Mineral GH+ (for Caridina) and Salty Shrimp Sulawesi Mineral 7.5 (for Sulawesi shrimp). For Neocaridina, Seachem Equilibrium works fine. The products are powder blends of calcium, magnesium, potassium, and trace elements; you dissolve them in RO water in measured amounts to hit a target TDS. A typical dose is 1 scoop (about 2 grams) per 10 liters of RO water to reach TDS 150 ppm. The math is simple and the results are repeatable, which is why shrimp breeders rely on RO + Salty Shrimp instead of tap water.

RO water is also useful for diluting tap water. If your tap is TDS 400 and you want TDS 200 for shrimp, mix 50/50 RO and tap. If your tap is TDS 250 and you want TDS 100 for Crystal Red shrimp, mix 60/40 RO and tap (60% RO, 40% tap). The exact ratios depend on your tap's TDS and your target TDS, but the principle is the same: RO is zero, so any mix with tap is a predictable linear dilution.

TDS Creep — The Silent Killer

TDS creep is the gradual rise in TDS over time in a tank topped off with tap water. Here is the mechanism: when water evaporates from your aquarium, only pure H₂O leaves — every dissolved solid stays behind. If your tank is at TDS 200 and 10% of the water evaporates, the remaining water is now at TDS 222 (same solids, 10% less water). If you top off with tap water at TDS 200, you add new solids along with the replacement water — the tank ends up slightly higher than 200, depending on exact mixing. Repeat this every week for six months, and your TDS can climb from 200 to 350 without you doing anything obviously wrong.

TDS creep is invisible to a fish keeper who never tests TDS. The fish look fine for months, then start dying or failing to breed, and the keeper cannot figure out why. A TDS test reveals the problem instantly — the number has crept up to 350 or 400 ppm, far above what the fish or shrimp were acclimated to. The fix is straightforward: do larger water changes with tap water (or RO if you want to lower TDS further), and switch from topping off with tap to topping off with RO water. RO top-off stops TDS creep completely because RO has zero TDS — you are replacing pure water with pure water, and the solids stay at their current concentration.

TDS creep is most severe in small tanks (where evaporation is a larger fraction of total volume) and in tanks with high surface agitation or open tops (where evaporation is faster). A 5 gallon nano tank with an open top can lose 10% of its volume per week to evaporation, which means TDS can climb 50% in six months without intervention. A 55 gallon tank with a glass lid loses 1 to 2% per week, so TDS creep is barely noticeable. The smaller the tank, the more important RO top-off becomes.

When to Worry About TDS

For fish-only tanks, TDS is a "nice to know" rather than a "need to know." Most community fish tolerate a wide TDS range (100 to 500 ppm) without obvious problems, and you can keep them for years without testing TDS at all. Where TDS matters for fish is in three specific situations: breeding (many fish need specific TDS ranges to trigger spawning), acclimating wild-caught fish (which may come from very low-TDS water and shock when moved to high-TDS tap), and diagnosing mystery health issues (a creeping TDS is a common cause of "fish just stopped thriving" cases).

For shrimp tanks, TDS is essential. Test weekly at minimum, daily if you are breeding. Track the trend over time — the absolute number matters less than whether it is stable, rising, or falling. A stable TDS at 250 ppm is healthier for shrimp than a swinging TDS between 150 and 300 ppm. Sudden TDS changes from a water change with mismatched water can shock shrimp and trigger molt failures. Always match new water TDS to tank TDS within 50 ppm when doing water changes on shrimp.

For planted tanks, TDS is useful for tracking fertilizer buildup. Every dose of liquid fertilizer adds TDS. If you dose daily and never do water changes, TDS climbs steadily as fertilizer accumulates. A planted tank running at TDS 600 ppm is probably over-fertilized — the plants cannot absorb nutrients that fast, and the excess fuel algae. Test TDS before and after water changes; if TDS is climbing despite weekly 30% water changes, you are dosing too much fertilizer. Cut the dose in half and retest in a month.

How to Lower TDS

The only effective way to lower TDS is to mix your tank water with lower-TDS water — which in practice means RO or distilled water. A 50% water change with RO water (TDS 0) on a tank at TDS 400 will drop the tank to TDS 200. A 30% water change with RO on the same tank will drop it to about TDS 280. The math is linear: the new TDS equals the weighted average of the remaining tank water and the new water. If your tap water is lower TDS than your tank (because of TDS creep), a regular water change with tap will also lower TDS — but only down to the tap water's TDS, not below.

For shrimp and soft-water fish, the standard method is to do water changes with a mix of RO and remineralizer. Mix RO water with Salty Shrimp to your target TDS in a separate container the day before a water change, let it come to temperature, then use it for the change. This gives you precise control — you can hit TDS 150 every time, regardless of what your tap water does. Shrimp breeders do this religiously; the consistency is what produces consistent shrimp health and breeding.

Do not try to lower TDS with chemicals or filter media. Products that claim to "remove TDS" or "soften water" generally do not work as advertised — they might bind some specific ions (e.g., a water softener pillow exchanges calcium for sodium, which does not lower TDS, it just changes the ion composition). The only filter media that actually lowers TDS is an RO membrane or a deionization (DI) resin, both of which are point-of-use systems plumbed into your water supply, not in-tank additives.

How to Raise TDS

If you are running RO water and need to raise TDS, the standard methods are Salty Shrimp (for shrimp), Seachem Equilibrium (for fish or shrimp), or crushed coral in the filter (for hard-water fish). For shrimp, the standard recipe is 1 level scoop (about 2 grams) of Salty Shrimp Bee Shrimp Mineral GH+ per 10 liters of RO water to reach TDS 150 to 200 ppm. For fish, 1 tablespoon of Seachem Equilibrium per 20 gallons raises TDS by about 50 to 75 ppm and GH by about 3 dGH. For hard-water setups, a mesh bag of crushed coral in the filter slowly dissolves and raises TDS, GH, and KH simultaneously.

Always remineralize RO water before adding it to the tank — never add pure RO to an established tank expecting the tank's existing solids to be enough. The RO water will dilute the tank's TDS, GH, and KH proportionally, which can shock fish and crash shrimp molts. Mix the RO with remineralizer first, test the TDS, match it to your tank's TDS within 50 ppm, then do the water change. This is the difference between a safe water change and a shrimp-killing mistake.

The cheapest way to remineralize small amounts of RO water for fish is baking soda (for KH) plus Equilibrium (for GH). For shrimp, skip the DIY and buy Salty Shrimp — it has the right Ca:Mg ratio and trace element blend that DIY cannot easily match. The $15 you spend on Salty Shrimp lasts months in a small shrimp tank and prevents far more than $15 in dead shrimp.

Frequently Asked Questions