We have acquired a used tank from some family members, and I am unsure of how to clean it properly. As far as I know, it used to house some freshwater fish. We are going to use it for our semi-aquatic turtles. What is the best way to clean it and to kill any icky things that still may be there? Should I go and buy something at the pet store to do it, or are there any good household tips? Thanks!