💧 Dilution to Target TDS Calculator
Plan aquarium dilution from current TDS, target TDS, tank volume, replacement-water TDS, source blends, evaporation correction, max-change limits, and remineralizer offset.
Use the TDS of the water going into the tank before any after-change offset.
No extra TDS is added after dilution.
Dilution Breakdown
| Aquarium use | Common TDS range | Dilution note | Change caution |
|---|---|---|---|
| Caridina shrimp | 90 to 160 ppm | Use low-TDS water and remineralize to the final target. | Keep single-stage changes small and retest often. |
| Neocaridina shrimp | 180 to 300 ppm | Blending tap with RO can land in range without a large swing. | Avoid sudden drops after evaporation spikes. |
| Planted community | 150 to 350 ppm | Track fertilizer and mineral offsets separately from dilution. | Large changes can alter KH and GH at the same time. |
| Softwater fish | 50 to 180 ppm | Lower TDS gradually with RO-heavy replacement water. | Stage reductions when current TDS is far above target. |
| Hardwater or rift setups | 300 to 600 ppm | Raising TDS usually needs mineralized replacement water. | Confirm GH and KH, not just meter ppm. |
| Common tank | Dimensions | Volume | 20% change | 35% change |
|---|---|---|---|---|
| 10 gallon | 20 x 10 x 12 in / 51 x 25 x 30 cm | 10 gal / 38 L | 2.0 gal / 7.6 L | 3.5 gal / 13.2 L |
| 20 long | 30 x 12 x 12 in / 76 x 30 x 30 cm | 20 gal / 76 L | 4.0 gal / 15.1 L | 7.0 gal / 26.5 L |
| 29 gallon | 30 x 12 x 18 in / 76 x 30 x 46 cm | 29 gal / 110 L | 5.8 gal / 22.0 L | 10.2 gal / 38.4 L |
| 40 breeder | 36 x 18 x 16 in / 91 x 46 x 41 cm | 40 gal / 151 L | 8.0 gal / 30.3 L | 14.0 gal / 53.0 L |
| 75 gallon | 48 x 18 x 21 in / 122 x 46 x 53 cm | 75 gal / 284 L | 15.0 gal / 56.8 L | 26.3 gal / 99.4 L |
| Source blend | Example inputs | Resulting replacement TDS | Best use |
|---|---|---|---|
| Pure RO/DI | 0 to 10 ppm source | 0 to 10 ppm | Lowering TDS or replacing evaporation minerals. |
| 25% tap / 75% RO | 240 ppm tap and 5 ppm RO | About 64 ppm | Softwater dilution with some mineral carryover. |
| 50% tap / 50% RO | 240 ppm tap and 5 ppm RO | About 123 ppm | Moderate target water for many planted tanks. |
| Remineralized RO | 5 ppm RO plus product offset | Offset-driven | Targets where GH and KH matter more than tap blend. |
| Change size | Mass-balance effect | When it fits | Calculator use |
|---|---|---|---|
| 10% | Moves 10% of the gap toward replacement TDS | Very sensitive tanks or first test change. | Compare against the exact plan in the scenario grid. |
| 20% | Moves 20% of the gap toward replacement TDS | Common maintenance-sized correction. | Often fits shrimp and planted systems. |
| 35% | Moves 35% of the gap toward replacement TDS | Faster correction when livestock and hardness allow it. | Use as a max-change cap if appropriate. |
| Exact | Solves directly for target TDS after offset | When one change is inside the max limit. | If too large, the stage plan splits it. |
Water testing for total dissolved solids will indicate mass of minerals present in the water. It has nothing to do with biological quality of that water.
Perhaps you test it and number is too high; then you grab a bucket and swap out the water. This instinct are dangerous because your livestock are not just looking for dissolved substances, but for specific ions like magnesium and calcium. Blindly chasing a lower number by changing the water can absorbs shock to your fish through rapid osmotic changes or even crash your pH. Better to gently nudge chemistry in the direction it needs to go.
Why You Should Use a Water Calculator
Calculating how much water need to be replaced is simple math, but easily botched if performed in your head. You must consider amount of water already in tank (volume), the mineral concentration within that volume (mineral concentration), and the concentration of what you’re replacing that with (replacement water). In other words, this is a simple mass balance equation. The thing is, unless you’re good at juggling equations in your brain whilst removing substrate waste, you probably don’t want to mess with all the numbers yourself.
The calculator above do it for you. Just plug in your current TDS value, your target value, and your source water characteristics and it’ll tell you precisely how much replacement water you’ll need.
Pre-calculating your change size without accounting for evaporation is one of most common errors made by hobbyists. As water evaporates out of the tank, the dissolved minerals remains. This creates a more concentrated solution inside the tank. That increase the TDS but doesn’t add anything else to the tank. Running this increased TDS value through your dilution plan will result in overestimating how much water needs to be removed. The result: you change out too much water and it kill off the good guys (beneficial bacteria). Accurate correction for evaporation first sets the baseline back to “normal.” Then, your dilution math will represent real mineral levels instead of artificially high concentrations.
Overly aggressive water changes can also be shocking to system. Large drops in ion concentration overnight can be lethal to many sensitive livestock like discus or even shrimp. You can specify an upper limit for the percentage change allowed at each step. If you’re starting with parameters very different from your goal, the calculator breaks it down into several stages. It’s not just good for fish’s comfort level; it also gives the biological filter time to adjust to changing ionic strength.
Hardier community tanks may be able to absorb larger percentage differences whereas more sensitive tanks (e.g., discus) might only tolerate a twenty percent change. That knowledge require experience which software won’t have.
Blended source waters adds more complexity. Most of us don’t get perfectly hard/soft water out of the tap. Instead, we either mix it with our mains supply or use an RO product and mix that into something else halfway through. The calculator allows you to input two water sources and their contributions so you can maintain consistent water chemistry. It’s important because if you make a small change to your blend ratio each week then your TDS isn’t going to stay on target. Knowing exactly what percent of each source goes into the tank helps ensure each water change stays in the same chemical zone. This minimizes long term variability.
The other unique variable is Remineralizers. By their very nature, they create mass that didn’t exist prior to the dilution event. Dosing anything to raise CO2, GH, KH, etc. Adding these supplements will increase final TDS. You would of overshoot your target if you don’t take this offset into account. You can account for the expected ppm increase of your additives so that the predicted result on test strip matches what actualy happens in the tank. It makes you consider whole cycle of water (bucket > tank > test strip).
Finally, the quest for the right TDS is more about keeping it in a good range but stable than seeking perfection for perfection’s sake. This isn’t about making things precise just because we can. This is about consistency. Plan out your dilution ratios ahead of time with help of the calculator. Take the guess work out of it. Stress the ecosystem less. Stop responding when you see a reading that sends you into panic mode. Instead, manage the water parameters intentionaly. Keep those minerals in their place. Let the livestock be happy. This turns water changes from high stakes experiment into something routine; a necessary part of maintenance. That’s the distinction between having fish and creating an aquatic environment.
