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You are likely having some sort of bacterial growth due to the new sand. Try and siphon it out and see how long it takes to grow back. If you could tell us your parameters that might help to narrow down or exclude some possible culprits. Also, how long did it take to appear and how rapid is the growth? How long has it been present? Is it spreading, receding or just kind of in stasis?
Let's get this solved. There are a ton of very knowledgable people here to help.
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