Water After Overwintering: Reuse or Refill the Pool?

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schlesos

New member
Apr 15, 2025
3
Zurich, Switzerland
Pool Size
57000
Hi everyone,

We have a pool with a capacity of approximately 57 m³. Last year, we completely replaced our old filtration and water treatment system and have been using a modern saltwater chlorination system (salt electrolysis) since then.

In previous years, we would start each swimming season by refilling the pool entirely with fresh water. For overwintering, we drained about one-third of the water – down to just below the return jets.

Thanks to the new system, the water is currently much clearer and shows significantly less algae growth or discoloration. The current water values are as follows:
  • Salt level: approx. 700 mg/l
  • pH level: approx. 6
  • Chlorine level: low, but still within an acceptable range
Our question:
Can we safely reuse the existing water for the upcoming pool season, or would you recommend a full refill?
(We're concerned that otherwise we might end up using excessive amounts of salt, magnesium chloride, pH adjusters, and chlorine - or that it could negatively impact the new system. A photo of the current pool condition is attached.)

Thanks in advance for any insights or advice! 😊
 

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Welcome to TFP!

There is rarely a need to replace water in a pool unless there are significant chemical or contaminant issues. Not normal issues. Excess calcium or a flooded pool from high water are the usual rare cases. A pool that is black from algae and decomposition and full of leaves and frogs can be recovered easily with chlorine and a little patience.

This is my 20th season with my ~80,000 liter pool and the only water I've ever added is to make up for evaporation during the summer.

You may want to check your testing method. It's very unlikely that your pH is as low as 6. You generally have to work pretty hard adding a lot of acid to the water to get the pH below 6.8. Pool test kits seldom measure below 7, and I've never seen one that measures below 6.8.
 
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Hi @JohnT

Thanks for your answer.
We have a saltwater electrolysis system that ideally operates between 1,200 and 1,800 mg/l during normal use.
In October 2024, we shut down the system for the winter, and the water remained in the pool over the entire off-season.
Now, the salt level has dropped to 700 mg/l, which is clearly too low.
We shut down the system in October 2024 and left the water as it was back then, without adding any additional chemicals to the pool.

Current status:
We’ve run the pool robot, which more or less cleaned the debris off the pool floor. However, after that, the water became quite cloudy. Now, we have some marks at the previous water level, and the cloudiness has mostly settled - I believe everything has settled back to the bottom.

Question 1:
From what I gather from your reply, it seems we can continue using the existing water, as long as we clean the walls and floor without using any chemical products - is that correct?

Question 2:
Besides adding salt and magnesium chloride, what other products or treatments should we add to the pool to get the saltwater electrolysis system running properly again without causing damage, and to bring the water values back into balance?
(We plan to top off the pool with approx. one-third fresh water.)
 
The best thing to do is to follow the SLAM Process using liquid chlorine as the sanitizer. Once you complete that, then you can add stabilizer and salt as needed and resume operating your SWCG.
 
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