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When I enlarged the picture, it looked like there were many round pieces that were fungused over. If you are feeding a micro pelleted food, it's that. If not, what kinds of fish are in the tank?
Colin & Andy make good points re fungus growing on food. In the beginning, I overfed my axotles. The uneaten pellets quickly came to resemble eggs to the untrained eye. This pair was previously owned by a physician who got them for his kid who lost interest. He told me they were a boy & a girl & laid eggs. Not! Two boys! Too many pellets.