If you don't have loads of plants, this will be a problem. It is nitrifying bacteria trying to find a foothold. My bet is that you are not completely 'cycling' your tank for nitrifying bacteria. It will also cause a white scum on top of the water. Sometimes, the tank will be cloudy. It may not hurt your fish, but it will look bad. I offer this: Raise the temperature of your tank to 78 degrees. Or, about 5 degrees higher than it is. This has cleared it up for me before. But remember, it is only temporarily masking the fact that you don't have enough nitrifying bacteria with a foothold. So your filter is not 'old' enough. So many reasons, but, chlorinated tap water cause you to lose nitrifying bacteria footholds, so make sure you dechlorinate your tank water properly. If you are doing that, then you have too many fish for the amount of bacteria. Don't worry, it will catch up, but you'll have to endure the 'ugliness' of the scum until it does. OR, you could over abundantly add plants and fertilize them properly, and not worry about your filter ability. See my tanks and website.