Dont know if its the right spot but I need to go so I dont have time to search. I have a poluted tank from my water its horrible water and killed off half my african tank. Ne ways I have sand in the tank.... if the water has been in there for awhile and I took out all the water and put new water in, would I have to replace the sand to be sure that water ever caused the death of my fish doesnt stay in the tank as it might soak up or stay inthe sand??