Dusty water?

[Aqua13]

We have a 10 gallon tank. A six year old Columbian shark that we inherited passed about a month ago after just a month in the tank. The tank was cleaned and refilled andwas empty of fish since then but the filter was running. Six days ago, we got three little goldfish and all was well until today when the water looks cloudy and as if there are dust particles floating around the tank and on the top of the water. The fish appear in good health, and the test strips are ok, however, this is very concerning. Changed the filter but have not seen any real improvement. What is this and how do we fix it? Full disclosure, we are newbies so help in layman's terms please! Thank you