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Hey guys!  Nice looking community.  This is my first season in aquaponics.  Here's my setup:
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15 gallon cement mixing tray with a 20 gallon sump.  Gravel is painstakingly washed bluestone.

Water goes in through the window to my tank inside:
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30 gallons.  Don't just my scape - I have to do some trimming.  I've got about 20 fathead minnows. 

 

Some of my plants are doing okay.  Some aren't.  The larger sprouts on the left are yu choy.  On the top and right, kale sprouts.  
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Everyone here started out in soil.  My "control group" sprouts that I left in soil are doing really well, but these little guys are struggling:
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What's going on here?  My first thought was nutrient deficiency.  I realize I don't have a lot of fish.  I could add some more, but it's not my favorite idea - not a fan of a crowded tank.  How about liquid ferts?  I've got a bunch of chems (potassium nitrate, potassium sulfate, mono potassium phosphate, magnesium sulfate, iron chelate) lying around, so I can mix my own if you guys see anything missing. 

Alternatively, I was thinking my drain cycle might be a bit fast - 5 minutes to fill, 1 minute to drain.  From conventional gardening, I'm used to interpreting this kind of yellowing as a sign of over watering.  But is that right for leafy greens?

Let me know what you guys think!  I appreciate any help you guys can offer.

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My guess, without knowing any other parameters, is the same as the aforementioned post.  Not enough nutrients.  From the looks of it, definitely not enough nitrate. 

 

As for adding the chemical fertilizer:  I don't run a single, combined aquaponics system where my water is recycled through grow beds.  Mine are separate, so I don't worry about additives and hydroponic fertilizer getting back into the fish water.  Somebody else will chime in on the chemical question pretty soon.  Might take a day....

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Have you tested for nitrates? I would guess you're not even close to having enough nutrients.

Additionally what nitrates you are producing are being used by the aquatic plants that are much more efficient at utilizing them.

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Yup!  You guys are right: no detectable nitrates.  Thanks for the quick response.  I'll catch some more fish for the setup soon.  In the meantime, any suggestions for how much/what type of chemical fert I can add?  Again, the setup is:
 

15 gallon growing area
20 gallon sump
30 gallon aquarium

20 2" fish.
Peppers, leafy greens, peas, tomatoes.  

For ferts, I have potassium nitrate, potassium sulfate, mono potassium phosphate, magnesium sulfate, iron chelate, and a mix of trace nutrients for aquariums.  I have a batch of fertilizer mixed up with these ingredients for aquaria.  

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This question might betray my poor grasp of chemistry, but would one expect potassium nitrate to trigger a nitrate water test?  After hearing from you guys that I have insufficient nitrate levels (but before Ravnis' post about potassium nitrate) I was adding small quantities of my potassium-nitrate rich home mixed fertilizer daily.  My plan was to stop adding ferts as soon as my testing kit detected nitrates. 

I have been at it for two days now, but this morning I decided to try a little experiment: I performed a nitrate test on a water solution that was basically 10% fertilizer.  No result.  I tried testing with a completely different test kit I picked up off Craigslist pretty recently, but still no nitrate detected.  What's going on?

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my best guess is they are being consumed quickly.  The API nitrate test is really picky about doing additions precisely according to instructions or it will give you a low - no reading.  Add solution #`1 and shake, then shake solution #2 vigorously for 30 seconds then add.  Shake the vial for one minute and then wait 4 minutes more to read.

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