RDWC Troubleshooting Guide — Common Problems and How to Fix Them

RDWC Problems? Here's How to Fix Them.

Recirculating Deep Water Culture (RDWC) is one of the most productive hydroponic methods available, but like any system, things can go wrong. The good news is that most RDWC problems have straightforward causes and fixes. This guide covers the most common issues UK growers encounter and exactly what to do about them.

Problem 1 — Yellowing Leaves

Most likely cause: pH out of range or nutrient deficiency.

In RDWC, pH drifts daily as plants absorb nutrients and water. If pH rises above 6.5 or drops below 5.0, plants can't absorb nutrients even if they're present in the solution — this is called nutrient lockout.

Fix:

  1. Check pH immediately. Target range: 5.5–6.0.
  2. If pH is out of range, adjust with pH Up or pH Down solution. Add slowly and recheck after 30 minutes.
  3. If pH is correct, check EC (Electrical Conductivity). Low EC = not enough nutrients. High EC = nutrient burn.
  4. If both are correct, identify which nutrient is deficient by the pattern of yellowing (lower leaves = mobile nutrient deficiency; upper leaves = immobile nutrient deficiency).

Problem 2 — Root Rot (Brown, Slimy Roots)

Most likely cause: Water temperature too high, insufficient oxygenation, or Pythium infection.

Healthy RDWC roots should be white and slightly fuzzy. Brown, slimy roots with a foul smell indicate root rot — one of the most serious RDWC problems but very treatable if caught early.

Fix:

  1. Check water temperature. Target: 18–22°C. Above 24°C dramatically increases Pythium risk.
  2. Add an air stone to the reservoir for additional oxygenation.
  3. Add beneficial bacteria (e.g. Hydroguard or similar) to the reservoir — these outcompete Pythium naturally.
  4. Remove any visibly rotten root material with sterilised scissors.
  5. Do a full reservoir change with fresh, pH-adjusted nutrient solution.

If your pump isn't keeping up with circulation, consider upgrading to our 25W Submersible RDWC Pump for larger systems.

Problem 3 — Slow or Stunted Growth

Most likely cause: EC too low, pH out of range, or insufficient light.

Fix:

  1. Check and correct pH (5.5–6.0) and EC for your growth stage:
    • Seedlings/clones: 0.8–1.2 mS/cm
    • Vegetative: 1.2–2.0 mS/cm
    • Flowering: 1.8–2.5 mS/cm
  2. Ensure roots have adequate air gap (2–5cm between water surface and net pot base).
  3. Check your lighting — insufficient light is a common cause of slow vegetative growth.

Problem 4 — Leaks

Most likely cause: Loose fittings or insufficient PTFE tape on threaded connections.

Fix:

  1. Identify the leak source — run the system with plain water and watch all connection points.
  2. Drain the affected section and dry the fitting.
  3. Re-wrap with PTFE Thread Seal Tape — 2–3 wraps clockwise around the thread.
  4. Hand-tighten, then a quarter turn with a spanner. Do not overtighten — this can crack fittings.
  5. Refill and recheck.

Problem 5 — Algae Growth (Green Slime in Reservoir)

Most likely cause: Light getting into the reservoir or pipework.

Algae competes with your plants for oxygen and nutrients, and can clog pipes and pumps.

Fix:

  1. Block all light from entering the reservoir — cover any transparent sections with black tape or opaque covers.
  2. Do a full reservoir change and clean all surfaces with a diluted hydrogen peroxide solution (3%).
  3. Check all pipe connections and lid seals for light gaps.

Problem 6 — Pump Failure or Weak Flow

Most likely cause: Blocked pump filter, pump undersized for the system, or pump failure.

Fix:

  1. Remove and clean the pump filter — mineral deposits and debris build up over time.
  2. Check all pipes for blockages or kinks.
  3. If flow is consistently weak, your pump may be undersized. Our 16W Pump suits smaller systems (4–6 buckets); the 25W Pump is recommended for 8+ bucket systems.

Problem 7 — pH Keeps Drifting Up

Most likely cause: Plants actively feeding (normal) or CO₂ off-gassing from the reservoir.

pH drift is normal in RDWC — plants absorb more anions than cations during vegetative growth, causing pH to rise. During flowering, the reverse often occurs.

Fix:

  1. Check pH daily and adjust as needed — this is a routine part of RDWC management.
  2. If drift is extreme (more than 1.0 pH unit per day), do a full reservoir change — old nutrient solution becomes unstable.
  3. Ensure your reservoir is covered to prevent CO₂ off-gassing and temperature fluctuations.

Prevent Problems Before They Start

The best RDWC troubleshooting is prevention. A well-maintained system with quality components rarely has serious problems. Browse our full range of RDWC systems and accessories — including pumps, fittings, and complete bundles — to build a setup that runs reliably from day one.

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