If it is chlorine system here is what I suggest
Clean/hoover out as much dirt/muck as possible. Do chlore shock until water is clear - can be repeated several times
Change 1/3rd of water each time - i.e. pump out 1/3rd with suction hose at the bottom of the pool. Refill then check for ph etc etc. This can also be repeated several times.
If your pool is 4 x 9 and average 1.5m deep (standard type of depth) then only 54m3 total water is needed.
This way you can avoid the liner moving - most important point as mentioned by Michael above
Good luck and enjoy the swimming
Phillip
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