Green slime is coating the inside of my reservoir and all my tubes. I've tried cleaning with bleach but it comes back in days. Is there a permanent solution? Does light leak cause this?

You've correctly identified that bleach isn't solving the problem - it's treating the symptom after the fact. Here's why algae keeps returning and how to stop it at the source.

Why algae grows in your reservoir

Algae need exactly three things: light, water, and nutrients. Your hydroponic reservoir provides all three in abundance. The only one you can realistically eliminate is light - and eliminating light access is the permanent solution.

Bleach kills algae on contact but leaves spores behind in every crevice of your tubing and reservoir walls. When you refill and expose the system to light again, the surviving spores germinate within 24–72 hours. That's why it comes back "in days."

Diagnosing your light leak

Every hydroponic reservoir has at least one light leak unless you've deliberately sealed all of them. Common sources:

• Net pot collars - even small gaps around the pot rim let light into the root zone
• Thin plastic lids - many DWC totes and NFT reservoirs use lids thin enough for light to penetrate, especially near grow lights
• Tubing entry points - holes drilled or punched for pump tubing create direct light pathways
• Fill ports - any access point that isn't covered when not in use

The test: take your system into a dark room and shine a flashlight inside while looking from outside. Any light that escapes = a light entry point in reverse.

The permanent fix: light exclusion

Reservoir wrapping. Wrap the exterior of your reservoir with reflective mylar tape or black-and-white poly sheeting (black side out to block light, white side in to reflect away from roots). Avoid paint - most latex and acrylic paints are not food-safe in aquatic contact and some off-gas compounds into solution.

Foam collars. Cut foam pool noodle sections to wrap tightly around any tubing where it enters the reservoir. Black craft foam also works and is easy to cut.

Net pot covers. Use black neoprene collars (sold specifically for DWC net pots) that completely block light through the pot opening. These are inexpensive and solve 80% of most growers' light leak problems.

Opaque lid tape. Run a layer of black electrical tape or gorilla tape over any seam in your reservoir lid.

One-time deep clean before restarting

Once you've sealed your light leaks, do a proper clean before restarting:

1. Empty and discard all nutrient solution
2. Scrub all surfaces with a 10% bleach solution (1 part bleach : 9 parts water) using a brush
3. Rinse thoroughly 3 times with plain water
4. For stubborn algae in tubing, run a hydrogen peroxide flush (3% H2O2, 30 mL per gallon of water) through the pump and tubes for 30 minutes
5. Final rinse with plain water, then refill with fresh nutrient solution

Ongoing maintenance

Once light is excluded, algae becomes a non-issue. You'll find residual light-green staining on reservoir walls - this is dead algae residue and is harmless. Do a full system clean between each grow cycle (every 4–8 weeks) as standard practice.

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This answer is reviewed against published horticultural research. It is not a substitute for advice from a certified horticulturalist.