Two identical 20-litre tubs, twelve butterhead lettuce seedlings, one question that gets argued to death in every hydroponics forum: when the water level drops, do you refill with plain water or more nutrient solution?

I ran both side by side for four weeks and logged EC every evening. The numbers settled the argument faster than any forum thread ever has.

What Was the Setup?

Both tubs were basic deep water culture: 20 L of solution, an air pump pushing 4 L/min through a stone, six Lactuca sativa 'Buttercrunch' seedlings per lid, under the same 240 W LED panel running 14 hours a day at about 25 cm height. If you have never built one, the layout matches the standard build in our DWC guide.

Starting solution in each: a two-part liquid nutrient mixed to EC 1.4 mS/cm and pH 5.9 at 20°C water temperature.

The single difference. Tub A got topped off with plain dechlorinated tap water (my tap reads EC 0.3) whenever the level fell 2 to 3 L. Tub B got topped off with fresh full-strength solution at EC 1.4. Everything else, including pH corrections, stayed identical. Each tub lost 2 to 3 L roughly every four days once the plants hit their stride.

Which Tub Drifted, and How Fast?

Both drifted, in opposite directions, and the direction is the whole story.

Tub A (plain water) sagged slowly downward: EC 1.4 on day 1, then 1.32, 1.27, 1.24 at each weekly reading. The lettuce never noticed. Leaves stayed flat, green, and crisp, and by day 28 the six heads averaged 142 g each.

Tub B (nutrient top-offs) climbed relentlessly: 1.4, then 1.66 by day 7, 1.95 by day 14, 2.28 by day 21. The plants absolutely noticed. Growth was quicker for the first ten days, then the leaf margins on four of six heads developed the crisp brown edges of tip burn, and by day 28 the heads averaged 128 g with visibly cupped inner leaves. I had to fight pH too; Tub B needed correction twice as often because concentrated solution swings harder.

The mechanism is simple once you see it in numbers. Lettuce transpires far more water than it removes nutrient salts, so every litre that leaves Tub B as vapour gets replaced by a litre carrying a full dose of salts. The salts have nowhere to go. Add solution to a shrinking reservoir and you are stacking concentration on concentration.

When Does Plain Water Stop Being Enough?

Plain water top-offs are the right default, not a forever rule. Three situations change the answer:

Long gaps between refreshes. After 2 to 3 weeks of top-offs, the nutrient ratios inside the reservoir are skewed no matter what the total EC says, because plants strip some elements faster than others. Nitrogen and potassium go quickly; calcium and magnesium lag. The fix is not a smarter top-off, it is a full drain and remix, the schedule I lay out in when to change hydroponic nutrient solution.

Heavy feeders in fruit. A tomato in week 8 pulls nutrients hard enough that EC can actually fall between top-offs. If your meter shows EC dropping 0.2 or more below target while the level also drops, top off with half-strength solution instead of plain water. That happens with fruiting crops, almost never with leafy greens.

Very hard tap water. My tap at EC 0.3 is workable. If yours reads 0.6 or higher, every plain-water top-off imports a meaningful load of carbonates that push pH up and crowd out the nutrients you actually mixed. Run the comparison in tap water vs RO for hydroponics before blaming your fertiliser.

What Should Your Top-Off Routine Look Like?

Here is the exact routine I settled on after this test, and it takes under five minutes per session:

  1. Mark your full line. A strip of tape on the outside of the tub at the 20 L mark. No guessing.
  2. Top off with plain, dechlorinated water whenever the level drops 10 to 15 percent. Match the water temperature to the reservoir within a few degrees; a 5°C cold shock to the roots costs you a day of growth. Cool reservoirs also hold more oxygen, which I measured separately in dissolved oxygen in hydroponics.
  3. Check EC and pH after each top-off, once the pump has mixed things for 15 minutes. You are watching for trend, not chasing a single number. Drift of 0.1 per week is normal; 0.3 in a week means check your meter calibration or your tap water.
  4. Log the litres you add. When cumulative top-offs equal the reservoir volume, 20 L added to a 20 L tub, drain everything and mix fresh. That usually lands at the 2 to 3 week mark for greens.
  5. Refresh fully every 2 to 3 weeks regardless of how good the numbers look, because ratio skew is invisible to an EC pen.

A passive Kratky jar plays by different rules; the falling level is the design, and you never top off at all, as covered in the Kratky method guide.

Frequently Asked Questions

Q: My EC keeps rising even though I top off with plain water. What is going on?

A: Check your tap water first; anything above EC 0.5 adds salts with every refill. Second suspect is evaporation from an uncovered reservoir, which concentrates solution without any drinking involved. Cover the tub, and if tap EC is high, switch top-offs to RO or rainwater.

Q: Can I just top off forever and never do a full nutrient change?

A: With greens you can push it 4 to 5 weeks before deficiencies show, but the crop pays for it in the last fortnight. Individual elements deplete at different speeds, so the mix drifts out of balance while total EC still looks fine. A full remix every 2 to 3 weeks is cheaper than a stunted harvest.

Q: Should I top off with pH-adjusted water or adjust after adding?

A: Add first, mix 15 minutes, then adjust the whole reservoir. Pre-adjusting a small jug of low-EC water is unreliable because plain water barely buffers; its pH swings the moment it hits the reservoir. One correction after mixing wastes less pH-down.

Q: Does the plain-water rule apply to coco or media beds fed from a reservoir?

A: Yes, and it matters more. Media holds a salt reserve that builds between feeds, so refilling a drip reservoir at full strength compounds two accumulation problems at once. Plain-water top-offs, plus a fortnightly flush of the media itself, keep run-off EC in range.

The reservoir tells you what it needs if you read the two numbers together: level says how much to add, EC says what to add, and for leafy crops the answer to the second question is almost always plain water. Keep a log for one full cycle and the pattern of your own system becomes obvious within a fortnight. To plan feed strength for the next remix, run your numbers through our free fertilizer calculator.