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Brian6895
Diana3118
It looks like a sponge.
Maria6659
It looks like cyanobacteria or diatoms. Something is off with the water treatment.
Katie5500
Typically, sponges have openings through which the water pumped through the sponge exits, but here it is solid jelly. Cyanobacteria are capable of photosynthesis, but there are no visible bubbles of oxygen in the photo; diatoms cannot exist in such a thick layer in an aquarium, as they build their skeletons from silica, and there wouldn't be that much available. I lean towards the idea that this is a colony of some bacteria. This is my opinion.