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.
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.