How to Use This Calculator
- 1
Enter Tank Parameters
Input your tank volume in gallons, current TDS reading (ppm) from your TDS meter, your target TDS (ppm), and the TDS of your RO/DI replacement water (0 ppm for pure RO). Click Calculate to see results.
- 2
Review Results & Insights
Check the RO Water to Add (gallons), Water Change %, TDS Reduction, Resulting TDS, and Dilution Efficiency cards. The TDS Reading color card shows your current level. The Insights panel shows the metric equivalent in liters, dilution efficiency per gallon, and a staging recommendation for large changes.
Example Calculation
An aquarist has a 55-gallon tank with a current TDS of 400 ppm and wants to reduce it to 200 ppm using pure RO water at 0 ppm.
Tank Volume (gal)
55
Current TDS (ppm)
400
Target TDS (ppm)
200
RO Water TDS (ppm)
0
Results
RO Water to Add
27.50 gal
Water Change
50.0%
TDS Reduction
200 ppm
Resulting TDS
200 ppm
Efficiency
7.27 ppm/gal
Tips
Calibrate Your TDS Meter Before Every Session
A miscalibrated meter can throw off your calculations by 50+ ppm. Use a 342 ppm calibration solution and re-test monthly — replace probes annually for consistent accuracy in 2026.
Stage Large Changes Over Multiple Days
If the calculator recommends over 50% water change, split it into 2-3 sessions over consecutive days. For example, a 50% change on a 55-gallon tank means 27.50 gal — do two changes of about 13.75 gal each to reduce fish stress.
Monitor RO Membrane Output
A new RO membrane produces 0-5 ppm water, but aging membranes creep up to 20-30 ppm, reducing dilution efficiency. Test your RO output weekly and replace membranes when TDS exceeds 10 ppm for best results.
Match Target TDS to Your Species
Crystal shrimp thrive at 80-150 ppm, community fish at 150-300 ppm, and African cichlids at 300-400 ppm. Use the Resulting TDS card to confirm your target matches your species' ideal range.
Precision Purity: Your Aquarium TDS Calculator
The Aquarium TDS Calculator helps aquarists calculate exactly how much RO water to add to reach a target TDS level.
Enter your tank volume, current TDS reading, target TDS, and RO water purity to get the precise water change volume, percentage, and dilution efficiency — essential for keeping sensitive species healthy in 2026.
Why Total Dissolved Solids (TDS) Management Matters
Total Dissolved Solids is a critical water parameter for aquarists keeping sensitive species like discus or crystal shrimp, which thrive in very soft, pure water.
High TDS can indicate accumulating pollutants, minerals, or organic compounds, leading to unstable water parameters, reduced fish health, and increased algae growth.
By accurately managing TDS, aquarists can prevent osmotic stress, enhance breeding success, and create a more stable environment that mimics natural habitats.
The Dilution Formula Behind TDS Reduction
The calculator uses a straightforward dilution formula to determine how much RO water replaces tank water to reach your target TDS:
Water Change (gal) = Tank Volume x (Current TDS - Target TDS) / (Current TDS - RO TDS)
This formula assumes the removed tank water is replaced with an equal volume of RO water.
When RO TDS is 0 ppm, it simplifies to a direct proportion of the TDS drop needed.
Reducing TDS in a 55-Gallon Tank: A Worked Example
An aquarist has a 55-gallon tank reading 400 ppm TDS and wants to lower it to 200 ppm using pure RO water at 0 ppm.
- Identify parameters:
- Tank Volume: 55 gallons
- Current TDS: 400 ppm
- Target TDS: 200 ppm
- RO Water TDS: 0 ppm
- Apply the dilution formula:
Water Change = 55 x (400 - 200) / (400 - 0)Water Change = 55 x 200 / 400Water Change = 55 x 0.5 = 27.50 gallons - Derived metrics:
- Water Change %: 27.50 / 55 = 50.0%
- TDS Reduction: 400 - 200 = 200 ppm
- Dilution Efficiency: 200 / 27.50 = 7.27 ppm/gal
- Volume in liters: 27.50 x 3.785 = 104.1 L
The calculator shows 27.50 gallons of RO water needed — a 50% water change removing 200 ppm of dissolved solids at 7.27 ppm per gallon efficiency.
Typical TDS Ranges for Different Aquarium Types
TDS benchmarks vary by aquarium type.
Freshwater planted tanks with CO2 injection typically target 100-250 ppm using remineralized RO water.
Sensitive shrimp tanks (Caridina species) prefer 80-150 ppm.
General community tanks naturally range 200-400 ppm depending on tap water and maintenance.
For marine aquariums, salinity (34-36 ppt) replaces TDS as the primary parameter, corresponding to roughly 34,000-36,000 ppm TDS equivalent.
These ranges help aquarists set appropriate target values in the calculator.
Frequently Asked Questions
What does TDS mean in an aquarium?
TDS stands for Total Dissolved Solids — the combined concentration of inorganic salts, minerals, and organic compounds dissolved in water, measured in parts per million (ppm). In aquariums, high TDS (above 400 ppm) can indicate accumulating pollutants or excessive mineralization that stresses fish.
What is an ideal TDS range for freshwater aquariums?
It depends on species. Crystal shrimp (Caridina) prefer 80-150 ppm, discus and soft-water tetras do best at 100-200 ppm, general community tanks thrive at 150-300 ppm, and African cichlids tolerate 300-400 ppm. Marine tanks measure salinity instead (34-36 ppt).
How does the dilution formula work?
The formula is: Water Change (gal) = Tank Volume x (Current TDS - Target TDS) / (Current TDS - RO TDS). For a 55-gallon tank at 400 ppm targeting 200 ppm with 0 ppm RO water: 55 x (400-200)/(400-0) = 27.50 gallons, a 50% water change.
What if my RO water is not 0 ppm?
Enter the actual TDS of your RO water. Higher RO TDS means you need more water to achieve the same dilution. For example, if your RO produces 10 ppm instead of 0 ppm, the same 55-gallon tank at 400 ppm targeting 200 ppm needs 55 x 200/390 = 28.21 gallons instead of 27.50.
How do I lower TDS without a water change?
Water changes are the most effective method. To slow TDS rise between changes, reduce feeding, vacuum gravel regularly, use activated carbon filtration, and add fast-growing plants that absorb dissolved nutrients. These are preventive measures, not substitutes for dilution.
What does the Insights panel show?
The Insights panel displays the volume conversion to liters, the dilution efficiency (ppm removed per gallon of RO water), and a staging recommendation — whether the change is safe to perform at once or should be split across multiple days to reduce fish stress.