If you have your own house in Thailand, and are not on direct city water, or pump city water into a holding tank, you will have a setup with a large water tank and a water pump.
You DO have to do maintenance on the water tank to ensure the tank is clean and the water stays fresh.
If this tank has been used a while then I would close the water inlet (so no more water enters the tank), use up as much of the water as possible that remains in the talk and then shut off the pump.
This will remove as much of the existing water in the tank as possible. Once you have used up as much water as possible, 0pen the cover of the water tank. Open the outlet flow at the bottom so all the remaining water can drain out.
Blast the inside of the water tank with a high pressure washer. This will wash all the dirt and grime out of the outflow valve.
You should install a basic filter before the tank (e.g. the municipal water going into the tank). This will save you from doing anymore tank cleaning in the future.
Close the outflow valve and water tank cover, turn on the pump and refill the water tank with fresh water.
You should put water tank care maintenance on a schedule so that you remember to clean it regularly.