This is the thing about salt water aquariums, everyone thinks that they are easy ... they are not. The hard part is solving the problems that they present to you, as you are currently experiencing now.
I am unable to solve your problem and if I did you would miss an important learning opportunity. You need to start thinking about what might be wrong and what you can do to validate or discredit your theory. The scientific method is your best friend.
I hope that you can work out what is causing your tank grief. :)
