We have a 10 gallon tank. We inherited a six-year-old Columbian shark who passed away approximately one month ago. The tank was emptied and cleaned and refilled with water and the filter left running. Six days ago we got three small goldfish and all was fine until yesterday when the water became cloudy and it appears as though there are dust particles floating all through the water and across the top. Changed the filter without much improvement. Test strips appear to say the water is fine. The fish also seem happy but this can't be good. What is this and how do we fix it? Full disclosure, we are newbies so advice in layman's terms would be much appreciated! Thank you in advance.