I'm new to aquaponics but have experience working on both sides of the system. I currently have a 150 gallon fish tank, 30 gallon swirl filter and 4 grow beds that can hold approximately 15 gallons of water each before using a bell siphon to return the water to the fish tank. I have this in my basement and use 2 tank heaters to keep my water temperature at +/-78f

Everything looks great as far as pH, DO and plants are growing faster than ever expected. But on my tank heaters I have found this stuff growing on them. No matter how much I clean them it grows back fast. Does anyone know what this stuff is?

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Thanks for your help!