500 Liters of what? Water, oil, gas?
Is this a new tank?
Is it steel, plastic?
What does this tank do?
If you do not want it to get dirty then do not use it. The first time you do use it any soilds or hevey metals will always settle to the bottom so never had the outlet sitting right on the bottom. Install a drain so you can drain off the sludge.
Most tanks are cleaned with a high pressure steam pressure washer.
If you want it easy and simple then just go buy a new tank. If it's old then it's going to be rusted out anyway and will start leaking soon.
Do a Google search on pressure tank.
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