Dear P S Krishnan,
That’s good you are constructing water storage tank Underground or open pound. kindly note some points which may help you.
1- If you are going for open pit in the land without any walls or base floor. there is a chance of seepage of water & evaporation. you cannot avoid the seepage but you can control the evaporation by spraying any oil on the top of the water.
2- You can construct a tarpaulin sheet lining in the pound base & walls, and can cover with the same tarpaulin sheet on top as your tank size is not so big
3- Do not go for concrete walls & base floor it will cost you a huge money. your point is correct it is not required plastering.
4- The Best one is use concrete floor bed that is big road base aggregate. floor bed thickness 8 inch & construct a granite wall of size 18 inches or 2 feet width. This also not required any plastering on the walls and floor. for safety you can use 6 mm steel rod on the top floor and make a plaster. do not forget to use the water proofing agent to mix in cement mortar. You can cover top with tarpaulin.
Best Regards.
Azhar2345