To determine a cause, let's start first with your tank:
How big is it?
How long has it been setup?
What is it stocked with?
What is your ammonia, nitrite, and nitrate testing at?
What is your phosphate testing at?
Are you using RODI water or tap water, or buying water from the store?
What kind of lighting do you use on the tank, and how long per day?
Is this a reef tank?
What equipment are you using? Filter? Skimmer?