TDS stands for total dissolved solids, and in an aquarium context it measures the combined concentration of all dissolved substances in your water, expressed in parts per million (ppm). Think of it as a single number that captures everything your water is carrying: minerals, salts, nutrients, and waste products. For freshwater aquarium keepers, especially those breeding shrimp, TDS is one of the most useful tools for monitoring water quality and keeping conditions stable.
What TDS Actually Measures
A TDS reading represents the total amount of dissolved ions and compounds in your water. The most common contributors are calcium, sodium, potassium, chloride, phosphates, and nitrates. In an aquarium specifically, fish waste, uneaten food, fertilizers, and minerals leaching from rocks and substrate all add to the total. Every substance that dissolves into your water column raises the TDS number.
Here’s the important nuance: TDS tells you how much is dissolved, not what is dissolved. A reading of 300 ppm could mean your water is rich in beneficial calcium and magnesium, or it could mean nitrate waste has been building up. Two tanks with identical TDS readings can have completely different water chemistry. This is why TDS works best as a monitoring tool for change over time rather than a standalone diagnostic.
How TDS Meters Work
Handheld TDS meters don’t directly weigh dissolved particles. They measure the electrical conductivity of your water, since dissolved ions conduct electricity, and then convert that reading into a ppm estimate. Electrical conductivity correlates with TDS at roughly 65 to 75 percent, so the meter is always giving you an approximation rather than a precise chemical analysis.
Most hobbyist meters are calibrated using a standard solution of 1,382 ppm. You dip the probe, confirm the reading matches the standard, and you’re set. These meters are inexpensive (typically $10 to $25) and accurate enough for aquarium purposes. The main limitation is that they can’t distinguish between types of dissolved solids. If you need to know your calcium or nitrate levels specifically, you’ll need separate test kits for those.
Ideal TDS Ranges by Species
TDS matters most for sensitive freshwater species, particularly shrimp. The two most popular groups have distinct requirements:
- Neocaridina shrimp (Cherry shrimp, Blue Velvet): 200 to 300 ppm. These are the hardier variety and tolerate a wider mineral range.
- Caridina shrimp (Crystal Red, Taiwan Bee): 100 to 200 ppm. These species need softer, more mineral-poor water and are far less forgiving of fluctuations.
Most common tropical fish are less sensitive to exact TDS numbers, but soft-water species like discus and many wild-caught tetras do better at the lower end of the spectrum, while African cichlids from the Rift Lakes thrive in hard, mineral-rich water with naturally higher TDS. For a typical community tank with hardy species, TDS is less critical than parameters like ammonia, pH, and temperature, but it still serves as a useful early warning system.
How TDS Relates to GH and KH
If you’ve tested general hardness (GH) and carbonate hardness (KH), you might wonder why TDS exists as a separate measurement. GH measures only calcium and magnesium ions. KH measures only carbonates and bicarbonates. TDS captures everything those two measure plus all the other dissolved substances: sodium, potassium, chloride, nitrate, phosphate, and more.
This means TDS can be high while GH stays low. If your water has a lot of sodium or potassium (from certain fertilizers or salt-based water softeners, for example), the TDS reading climbs but GH won’t budge, because those ions aren’t calcium or magnesium. Conversely, water with very high calcium and magnesium will show high readings on all three. The takeaway: TDS gives you the big picture, while GH and KH give you specific details about mineral content. For shrimp keepers, tracking all three provides the clearest view of what’s happening in the tank.
What Causes TDS to Rise
In a closed aquarium system, TDS almost always trends upward over time. Several things contribute:
- Evaporation: When water evaporates, it leaves all dissolved solids behind. The same amount of minerals concentrates into less water, so TDS rises even though you haven’t added anything. This is the single most common cause of creeping TDS in home aquariums.
- Fish waste and food: Biological waste breaks down into dissolved compounds. Overfeeding accelerates this.
- Fertilizers: Planted tank dosing adds potassium, nitrogen, phosphorus, and trace elements directly to the water column.
- Rocks and substrate: Some hardscape materials dissolve slowly into the water. If you place rocks in a bucket of clean water and the TDS rises over a few days, those rocks are leaching minerals. Limestone-based stones can push TDS above 400 ppm. Truly inert materials like lava rock or certain aquarium-specific stones won’t change TDS at all.
The evaporation factor is worth emphasizing because it catches many beginners off guard. If you top off an evaporating tank with tap water (which has its own dissolved minerals), you’re adding minerals on top of the ones already concentrating. Over weeks, this compounds into a significant TDS increase.
How to Lower and Stabilize TDS
Regular water changes are the most straightforward way to bring TDS down. Replacing a portion of tank water with lower-TDS water dilutes the accumulated solids. For shrimp tanks where precision matters, many hobbyists use water filtered through a reverse osmosis (RO) or RO/DI system, which strips nearly all dissolved solids and lets you rebuild the mineral content to an exact target using remineralizing products.
When topping off water lost to evaporation (as opposed to doing a water change), use pure RO or distilled water rather than tap water. Since evaporation removes only water and leaves minerals behind, replacing it with mineral-free water keeps TDS stable instead of letting it climb.
One quirk of RO systems that aquarium keepers should know about: the first water that comes out after the unit has been sitting idle often carries a higher TDS than normal. This happens because dissolved contaminants concentrate in the stagnant water sitting against the membrane. The fix is simple. Run the filter for 30 to 60 seconds and discard that initial burst before collecting water for your tank.
Why Stability Matters More Than a Perfect Number
Rapid swings in TDS are more dangerous to fish and invertebrates than a reading that sits slightly outside the “ideal” range. When TDS changes suddenly, the concentration of dissolved substances inside an animal’s body no longer matches the surrounding water. This creates osmotic stress, where water moves across cell membranes in the wrong direction or at the wrong rate. Fish experiencing this may lose color, breathe rapidly, become lethargic, or behave erratically.
Neocaridina shrimp can live perfectly healthy lives in a tank with TDS of 350 ppm if conditions are stable. Some hobbyists running tanks with limestone hardscape report TDS above 400 ppm with thriving shrimp and plant colonies. The problems start when TDS swings by 50 or 100 ppm in a short period, such as during a large water change with poorly matched replacement water. For sensitive species, matching the TDS of your replacement water to the tank water before doing a change prevents this kind of shock.
A practical habit is to test TDS once a week at the same time, log the number, and watch for trends. A slow upward drift tells you evaporation or waste buildup is outpacing your maintenance schedule. A sudden spike after adding new hardscape points to mineral leaching. The number itself is less important than the pattern it reveals over time.