- Record top-ups separately from fertilizer additions.
- Calculate a single constituent’s drain-and-refill balance.
- Recognize when EC cannot diagnose the solution.
Concentration dilution calculator
Replacement fraction = (initial − target) ÷ (initial − replacement).
Replace (% of volume): 40; Drain then replace (L): 40
One well-mixed drain then refill, with no new loading or uptake. Use the same constituent and reporting basis. This is not an emergency fish-water-change recommendation or an EC prediction.
01 / Keep an inventory
The reservoir is an inventory that changes through crop uptake, evaporation, leaks, harvest removal and maintenance. Its label capacity is not the amount of solution currently feeding the crop. Mark the working level and measure additions.
A top-up restores water; a nutrient addition changes dissolved mass; a replacement removes some of the old ion mixture. Combining all three under “refilled the tank” makes diagnosis difficult. Use separate log entries.
02 / A repeatable refill sequence
Check the level and sample before changing it. If the system is running low, restore the planned volume with suitable source water, mix and remeasure. Then decide whether the crop program calls for nutrients or a replacement.
A stable EC after many top-ups can hide ion imbalance. Oklahoma State’s EC and pH guide notes that sodium chloride accumulates through repeated water and nutrient additions and advises replacing the solution completely every two weeks; treat that interval as the source’s general advice rather than a universal rule. Fixed intervals such as “every seven days” or “after one reservoir volume” are defaults, not rules: track cumulative water additions and crop stage, and use periodic analysis when precision matters.
- Record starting level, temperature, EC, pH and crop appearance. Check for leaks if water loss changed abruptly.
- Add measured source water to the working mark. Let the tank and remote components mix.
- Remeasure EC/pH before dosing fertilizer. Account for source-water ions and the current nutrient program.
- If replacing solution, first switch off and unplug any submerged pump, heater or other water-contacting electrical equipment before the level drops below it, keeping the air pump and its line above the water as its backflow instructions require; then collect the old solution and manage disposal under local requirements. Clean deposits between batches as appropriate.
- Prepare and verify the replacement batch, restart, and confirm every outlet and pump intake.
Reading: Electrical Conductivity and pH Guide for Hydroponics ↗
03 / Partial replacement arithmetic
For a well-mixed drain-and-refill with no new loading during the operation, final concentration = initial × (1 − fraction) + replacement × fraction. If a measured ion is 120 mg/L and replacement water contains 20 mg/L, reaching 80 mg/L requires (120 − 80) ÷ (120 − 20) = 40% replacement. In 100 L that is 40 L.
This arithmetic applies to a specified measured constituent. EC need not mix perfectly linearly across different ion mixtures, and a dilution cannot correct the ratio of two nutrients that are both depleted. It is also not a recommendation to make abrupt large water changes around fish.
| Record | Example | Purpose |
|---|---|---|
| Starting inventory | 100 L; measured ion 120 mg/L | Defines dissolved mass. |
| Replacement water | 20 mg/L of the same ion | Avoids assuming pure water. |
| Target after refill | 80 mg/L | A chosen scenario, not a crop recommendation. |
| Calculated exchange | 40 L | One well-mixed drain then refill. |
04 / Investigate what changed
Rapid concentration increase with normal water loss can indicate a dosing error; rapid water loss can indicate a leak or overflow. A pH swing after a source-water change suggests examining alkalinity and the dosing history.
Keep batch labels and measuring tools until the crop is established on the new solution. If a calculation was wrong, the records should let you reconstruct the added mass rather than guessing at a correction.
FIELD QUESTIONWhy is replacement-water concentration part of the equation?
Removing old solution also introduces dissolved material in the replacement. Assuming zero when the replacement contains the same ion underestimates the remaining concentration.
Parts & buying criteria
Build your own parts & cost worksheet →
Showing aerated deep water culture. Quantities describe the teaching model. Specify real working volume, support, fittings and instruments for your installation.
01Nutrient reservoir1 · illustrated quantity+
Stores the measured nutrient solution beneath or beside the growing area.
Inspect: Compare the surface with the diffuser and the lower roots; freeboard is deliberate. There is no pump intake in this single-vessel example.
Maintain: Record level before refilling, keep light out and verify temperature, EC and pH.
Opaque, cleanable, intended-use container with volume marks and service access.
Check size, materials and operating conditions with your chosen supplier before ordering.02Lid & root support1 · illustrated quantity+
Holds the plant above the chamber or reservoir without pinching its crown.
Inspect: Separate the cover and inspect the basket or collar opening.
Maintain: Check fit as plants grow and keep unused openings covered.
Rigid compatible support with removable, correctly fitted baskets or collars.
Check size, materials and operating conditions with your chosen supplier before ordering.03Air pump & diffuser1 · illustrated quantity+
Delivers air below the water surface in this aerated DWC example.
Inspect: Trace the thin line from the dry pump to the diffuser. Bubbles are illustrative.
Maintain: Check actual delivery and dissolved oxygen; preserve backup air and prevent back-siphoning.
Rated output at the installed depth and diffuser resistance; accessible replaceable parts. Power it through ground-fault protection (GFCI/RCD) and keep the pump and its plug above the water line.
Check size, materials and operating conditions with your chosen supplier before ordering.Evidence beside the lesson.
Source checks: September 2026. The geometry, inspection exercises and worksheets are original teaching material. The named organizations have not endorsed or reviewed this site.
Published recipe rates and planning ranges retain their source context. Calculator equations are accounting tools; they do not predict uptake, yield, toxicity or safe stocking. Model dimensions, water speeds, roots and fish counts are illustrative. Verify species, crop, source water and product labels for a real system.
- University of Missouri ExtensionHydroponic Nutrient Solutions (G6984) ↗
Solution preparation, elemental nutrition and measurement.
- Oklahoma State University ExtensionElectrical Conductivity and pH Guide for Hydroponics ↗
Water analysis, salinity, pH and monitoring.
The next useful connections.
Nutrient mixing: from grams to elemental ppm
Scale two documented fertilizer examples, read the bag analysis and calculate what actually enters the final solution.
Interactive tool · 6 min + guided practice →Source waterSource water and alkalinity
Read a laboratory report, distinguish equivalent units from elemental nutrients and prepare water deliberately.
6 min + guided practice →Buying guidesChoosing EC and pH meters: range, resolution, calibration, probes
An EC or pH meter is bought against the solution band you must hold and the calibration routine you will keep, not against the digits on its display.
Interactive tool · 8 min + guided practice →