My EC is too high — how much water do I add?
Enough that the salt already in the tank, spread through the new volume, lands on the target. Dissolved salts don't leave when you add water — volume × EC is a quantity of salt that only draining or the plants can remove — so the arithmetic is conservation, with one term the simple formula forgets: the water you're adding isn't zero. Tap water at 0.3 mS/cm brings its own conductivity, and a target the tap can't get below is a target you reach with RO water or not at all.
20 gallons at 2.0 mS/cm, target 1.6, tap 0.3: 20 × 0.4 ÷ 1.3 = 6.2 gallons. With RO water (0.0) it's 5.0. The classic C1V1 = C2V2 gives the RO answer and quietly assumes the tank can hold it — which is the second thing the calculator checks.
Gallons of water by current and target EC
| Reservoir at | → 1.2 | → 1.4 | → 1.6 | → 1.8 |
|---|---|---|---|---|
| 1.8 mS/cm | 13.3 gal | 7.3 gal | 3.1 gal | 0 |
| 2 mS/cm | 17.8 gal | 10.9 gal | 6.2 gal | 2.7 gal |
| 2.2 mS/cm | 22.2 gal | 14.5 gal | 9.2 gal | 5.3 gal |
| 2.4 mS/cm | 26.7 gal | 18.2 gal | 12.3 gal | 8.0 gal |
| 2.8 mS/cm | 35.6 gal | 25.5 gal | 18.5 gal | 13.3 gal |
20 gal reservoir, tap water at 0.3 mS/cm. Bringing 2.8 down to 1.2 takes 36 gallons — more than the reservoir — which is a drain-and-refill, not a top-off.
The 20-gallon example above needs 6.2 gallons of water, and a 22-gallon tote has room for 2. So drain first: pull 3.2 gallons of the strong solution out, add 5.2 gallons of tap water, and the tote sits full at 1.6 — the calculator solves the two-step case whenever the one-step one overflows. Draining costs you nutrient you paid for; adding water before draining costs you nothing but wastes the drain. Do it in that order.
Water or nutrient? The number that decides
Every FAQ says "top off with plain water, because plants drink water faster than they take nutrient". True on average, and useless on any given day — sometimes they take both, and a reservoir topped off with water for three weeks quietly starves. Two readings settle it. If the tank went from 20 gallons at 1.5 to 16 at 1.7, it lost 4 gallons and 2.8 "gallon-EC" of salt; the plants therefore removed 4 gallons of something at 0.70 mS/cm. Top off with 4 gallons at 0.70 and the reservoir is back at 1.5 — exactly.
| 20 gal at 1.5 became 16 gal at… | Top-off EC | Top off with | Back to |
|---|---|---|---|
| 1.4 | 1.90 | stronger than the mix — they’re hungry | 1.50 |
| 1.5 | 1.50 | full-strength mix | 1.50 |
| 1.6 | 1.10 | weak feed at that EC | 1.50 |
| 1.7 | 0.70 | weak feed at that EC | 1.50 |
| 1.875 | 0.00 | plain water | 1.50 |
| 2 | — | readings disagree — re-measure | — |
Top-off EC = (V₁·EC₁ − V₂·EC₂) ÷ (V₁ − V₂). EC unchanged means balanced uptake — top off with the full mix. EC up to 1.875 means they took water only. EC above that can't happen from uptake or evaporation (neither adds salt) — one of the readings is off, or someone added nutrient.
The rule that falls out: top off with water only when the top-off EC is near zero. In hot weather under strong light it usually is; in a cool room with a young canopy it usually isn't. The calculator prints the number from your two readings and names the mix.
Drain and replace
The other way down, and the only way to remove what water can't: sodium, chloride, the imbalance left after plants take some ions faster than others. Swapping a fraction for fresh mix moves the EC a proportional part of the way:
| Reservoir at | Swap 25% for fresh at 1.2 | Swap 50% for fresh at 1.2 | Swap 75% for fresh at 1.2 | Swap 100% for fresh at 1.2 |
|---|---|---|---|---|
| 2.4 mS/cm | 2.10 | 1.80 | 1.50 | 1.20 |
| 2 mS/cm | 1.80 | 1.60 | 1.40 | 1.20 |
EC after = EC × (1 − fraction) + fresh × fraction. To take 2.0 to 1.6 with fresh mix at 1.2, swap 50%. A full change every one to two weeks is still the honest baseline — top-offs and partial swaps keep the EC right, not the recipe.
Why is my EC rising when I haven't added anything?
Because the plants took water and left the salt behind, and the smaller the reservoir the bigger the swing. A canopy that drinks a gallon a day is nothing to a 40-gallon tote and a quarter of a 5-gallon bucket:
| Reservoir | Starts at | After 1 day (−1 gal) | After 3 days |
|---|---|---|---|
| 3 gal | 1.5 | 2.25 | empty |
| 5 gal | 1.5 | 1.88 | 3.75 |
| 10 gal | 1.5 | 1.67 | 2.14 |
| 20 gal | 1.5 | 1.58 | 1.76 |
| 40 gal | 1.5 | 1.54 | 1.62 |
Water loss only, no nutrient uptake — EC × (volume before ÷ volume after). A 5-gallon DWC bucket that drinks a gallon a day climbs from 1.5 to 1.88 in one day and 3.75 in three; the same plant on 20 gallons moves to 1.76. Small reservoirs need daily top-offs not because anything is wrong, but because the arithmetic is unforgiving — which is the case for the biggest tote that fits.
How often to change it completely
Top-offs and partial swaps hold the EC where you want it, but EC is one number for a dozen ions, and plants don't take them evenly: potassium and nitrate go fast, calcium and sulfate slower, sodium from the tap not at all. A reservoir kept at a perfect 1.6 for a month by top-offs can be 1.6 of the wrong things. The honest baseline is a full change every one to two weeks — weekly for small reservoirs and fast crops, two for big totes in veg — and the drain-and-replace table above is what a partial change buys you in between. If pH drifts up day after day while EC stays put, that's bicarbonate from the tap accumulating, and it's the change date arriving early.
Why C1V1 = C2V2 is the wrong formula here
It's the right formula for what it's for: diluting a concentrate into a final volume of pure water. A running reservoir breaks both assumptions. The diluent isn't pure — tap water carries 0.2–0.6 of its own — so the simple version under-doses the water by that fraction every time; and the "final volume" isn't free to be anything, because the tote has a lid. The calculator's version keeps the tap's EC in the equation and checks the ceiling, which is all the difference between "add 5 gallons" and "add 6.2, and it won't fit, so drain 3.2 first".
Before you adjust anything
Stir, and read the meter at solution temperature if it doesn't compensate (the EC page corrects that). Check the volume honestly — a marked tote beats a guess, and a float valve makes the "volume then" reading a constant. Over 3 mS/cm, dilute before you diagnose: leaf edges that look like a deficiency are usually roots that can't drink. And when the top-off EC comes back negative, don't add anything — one reading is wrong, and the fix for a wrong reading is a second reading.
Sources: conservation of dissolved salts (EC linear in concentration over the hydroponic range — standard in every EC meter's specification); manufacturer reservoir FAQs (General Hydroponics, Bluelab) for the top-off and full-change guidance; stage EC bands as on the EC ↔ PPM page; bottle strength from the dosing calculator.