So I tested my water last night and my nitrates were about 20ppm and so I did a water change. Today I tested it again and it dropped but barely its about 15 ppm now. All that's in it is a peppermint shrimp and 5 blue leg hermits. Its a 10 gallon with live rock and sand. Why are my nitrates still high? Does it need more then a day or did I do something wrong?