What's going on here

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Guess...

 

[fishlover1478]

not another one of these threads lol

 

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Answer: It's actually what sometimes happens in very high nutrient water: Very dark/black slime oily growth has trouble holding on, and lets go, leaving white spots. The rocky textures hold on as good as can be expected, but can be further helped by maximizing light to 24 hours, and brushing clean every 3 days. Point is that high nutrients go from the water to the slime, but slime cannot hold on.