Aquaponic Gardening

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I'm running into an unidentified problem with the growth of leaf lettuce in my 100 gallon aquaponic system.  The system operates with a 15 minute flood and drain cycle and contains expanded clay pebbles and worms in the grow bed.

The grow light is a custom blue/red LED light that hangs approx. 1.5' above the top of the lettuce.  This discoloration seems to be happening on older leaves and starts around the edges.

Has anyone seen this before?  Thank you for your help!

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This is just my opinion I haven't ran into this problem before. I don't think its a lack of nutrients (fish). I would say that it has to do with the depth of your bed. Here why. Personally I've grown in bigger growbeds using less fish. You said you have Comet goldfish, that's what I use to kick my system off because they are cheap and produce 3x the ammonia as other goldfish breeds the same size. If your pH is really that high I would say that the growbed isn't cycling enough. By it not cycling at optimum efficiency I would almost bet that if you were to dig up your three worst lettuces and dig down into the bed you would find quite a bit of sludge build up, once you get down below 6 inches. Since you are only growing lettuce (roots only go down about six inches) no roots can get low enough to pick up them nutrients hanging around the bottom. The more sludge that builds up from the roots not cleaning the water adequately the more anaerobic your system becomes. The more anaerobic the system becomes the higher the pH gets. High pH isn't the cause its just a symptom.

If I'm right and there is a sludge buildup you can do one of two things as quick fixes. You could take out about three inches of clay pellets, or you could increase your cycles to 8 per hour. I hope this helps. I would say get a fresh water test kit, too.

Thank you for your advice.  I have the test kit and will start testing more regularly.  Will definitely also take a look at sludge buildup and explore adding to the fish population if necessary/ increasing cycle speed.  All things that I haven't considered so I really appreciate the suggestions!

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