Have you not filled it up yet? When I get a new tank I get water in it from other tanks and fresh water as soon as possible as well as adding the gravel to introduce the bacterial colonies as soon as possible. Also try using a filter from another tank for about a week you will find the tank cycles in no time! Thats what I did with my 50Gallon tank and it has been stable and no hassle ever since
Ben
edit... If it is for a Marine tank feel free to ignore me