If your 30,000 litre tank of water is not losing heat to the surroundings, then the power you need to apply to your tank is zero. While "zero heat loss" might or might not be realistic, it is important to understand this answer for this scenario.
If your 30,000 litre tank of water is losing heat to the surroundings (this could be by conduction through the tank walls, or airflow over an exposed surface (which to richen things up, might include some evaporative effects too)), then to hold the temperature of the tank constant you need to exactly replace the heat that is being lost.
It could be that what you really want to know is "how do I estimate the heat loss from a 30,000 litre tank". This is mostly of an engineering question, your best way to get a good answer might be to consult some engineering handbooks. The answer will depend on the size and shape of the tank, the conductivity of the tank's walls, whether or not the top is open or closed, whether the air surrounding the tank is still (so the heat transfer to the air is via natural convection only) or whether it is in motion (exposed to wind, fans, etc).