Short answer
Top off a hydro reservoir according to what its EC did since the last check. EC up: plain, pH-adjusted water. EC steady: solution at the reservoir's current EC. EC down: solution at full target strength. With no meter, plain or quarter-strength water is the safe default, and a full refresh every 7–14 days resets everything. Grower practice, built on basic uptake chemistry.
Why
Plants absorb water and nutrient ions at different ratios, and the ratio changes with stage, light and humidity [1]. When they drink more water than salts, the solution concentrates and EC rises. When they take more salts than water, EC falls.
A fixed-strength top-off ignores which is happening: full strength on a rising EC builds salts; plain water on a falling EC starves the plant.
EC is a total, not a recipe. Each element is taken up at its own rate, so the ion balance drifts even when EC reads correctly. That is why closed systems still need periodic full changes [2].
Fresh top-off water should be at reservoir temperature, 18–22 °C (64–72 °F). Warm top-ups raise root-rot risk [3].
The numbers
| EC change since last check | Top off with | pH |
|---|---|---|
| Rising (+0.2 mS/cm or more) | Plain water | 5.6–6.0 |
| Steady (±0.1 mS/cm) | Solution at current reservoir EC | 5.6–6.0 |
| Falling (−0.2 mS/cm or more) | Solution at full target EC | 5.6–6.0 |
| No meter | Plain or quarter-strength | 5.6–6.0 |
| Full change | Every 7–14 days | 5.6–6.0 |
DWC EC targets: 0.6–1.0 mS/cm seedling, 1.0–1.6 veg, 1.4–2.0 flower (ppm500 = EC × 500; ppm700 = EC × 700).
What to do
- Mark the full line and log EC and pH daily.
- Before topping off, compare EC with yesterday's reading.
- Mix the top-off to match the table, at reservoir temperature.
- Re-check pH after topping off; adjust within 5.6–6.0. Wear gloves and eye protection with pH-down acid.
- Change the whole reservoir every 7–14 days.
Common mistakes
- Topping off with full-strength nutrients every time, so EC climbs all week.
- Top-off water much colder or warmer than the reservoir.
- Skipping full changes because the EC "looks fine".
Related
- Full article: Deep water culture (DWC)
- Answers: algae in the reservoir · how long mixed nutrients keep
- Systems: RDWC · recirculating drip
- Terms: reservoir · EC
