Aquaponic Gardening

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I am fairly new to aquaponics and this my second summer trying it. Question is, there seems to be a slimy black film covering my clay pebbles. Is it fish poop, algae or mold? Don't know if it's toxic and I should be washing and starting all over? It is in a greenhouse, and it's just a small system (flood and drain) supported by about 8 goldfish in an a 21 gallon tank below.


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Is the black film covering the clay over the entire bed?  Or just where the water enters the bed.  Where you have wet media on the surface exposed to sun, you will get algae and I wouldn't worry about surface algae too much.


However if your entire bed is getting this algae film, you are probably flooding your bed too deep.  You generally want to leave the surface of your media dry as much as possible or many plants will suffer crown or stem rot and you will be feeding more of your nutrients to the algae than your plants.

It covers the surface only, was worried the black stuff was toxic. Thanks for letting me know veggies grown in here is safe to eat.


If it was mold I don't think it would look like slime to me, algae is likely to look glistening and slimy while mold tends to be more powdery.
I don't see black algae much, except where it's accumulated into a thick film with lots of dead cells.  Maybe if you had a picture?
I was simply kinda figuring that the black slime is really just very dark green.  And if it drys out between flood cycles you probably would see lots of dead algae sells getting darker as well.
Thanks to all for your response, here is the
You are flooding your bed too deep, I can see the wet mark on the side of the bed and it looks like the water is coming up over the media quite a bit.  You want the top surface of your media to stay dry so you either need more media in the bed or you need to lower your flood height so the top 1" (one inch) of media is above the flood height.
TCLynx is right but yea you're safe to eat your crops.

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