thank you very muchAre they more white in person than in the photos? And are they stationary?
If yes to both - I made a similar enquiry months ago and the consensus seemed to be that they were small sponges of some sort. Beneficial to the system overall.
Others way more knowledgeable than me will be along shortly, and I could very well be wrong, but happy to be corrected and learn.
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Jart's 3ft Shallow "Cube" Adventure