I'm not sure if it can be installed after a pool has been installed, try contacting a company who makes these hydrostatic valves and see if it can be done. good luck.
At the lowest point on the hot water distributing pipework system
There is no filter at the bottom of a pool. Perhaps you are looking at the hydrostatic valve which is there in case your pool is emptied and Will make sure that the pool wont float in ground water.
This is the hydrostatic valve. If the pool is emptied it is removed to relieve water pressure caused by the surrounding water table If not removed a pool can float out of the ground.
Any pool can have a hydrostatic valve and nothing has been made that cannot be broken eventually.
Very carefully
The Hydrostatic valve is usually on the lowest part of the pools floor. The hydrostatic valve is there to protect the pool from being lifted by ground water in the event that it is empty. it allows the ground water under the pool to get into the pool thereby stopping it from floating out of the ground on top of the ground water.
Aortic semilunar valve
A Hydrostatic valve is basically a plug that stop water coming down out of the pool, but when the water pressure under the pool gets higher then the pressure in the pool it will let water into the pool from underneath in order to stop the pool from floating in ground water.
If you have such a valve, it was installed as an aftermarket modification. Check with whoever installed it.
exspanion tank or relive valve
you will have to dig it out and put it back in the ground check for broken pipes and is at all possible install a hydrostatic valve so it wont happen again
A "relief valve" can only be accessed when the pool is empty, and no swimming pool should ever be left empty. The only exception is when absolutely necessary due to repair or resurfacing, and that should be completed within the shortest possible time. Any pool that is empty, and that has a relief valve, should have the valve removed for the entire time, regardless of the weather forecast. A "hydrostatic" relief valve opens and closes when needed as it attempts to equalize the pool water pressure with the ground water pressure. Human intervention is not involved because the pool is still filled. Hydrostatic valves are great in theory, but seldom work after a few years.