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How do I get rid of the cloudness in my fish tank?

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I just bought a fish tank not even a few months ago and I cleaned it all out when my fish batch of fish died and then I put more fish and I noticed the water keeps getting cloudy and there are black stains on the sides of the tank. I bought a albino cory catfish to clean it but I dont see that its doing its job. my tank gallon is 15 gallon and I have the filter that it came with on and running with the light.