tank cleaning

[mycat99]

i have a question about cleaning a very gross tank .the place i work for buys houses that are run down and the house we are starting on has a 7 foot salt water tank that was left behind .the tank is so bad that a filled 2 liter water bottle will not sink into tank because of scum .the over flow box and sump are almost solid waste .tank smells gross to .my boss said if i want it he sell cheap but would it ever be safe to use even after cleaning