refilling a fiberglass pool


In The Industry
Mar 29, 2007
Knippa, Texas
I am considering replacing my pool water, and since my pool is fiberglass, it must be filled with water at all times. I have heard about the "sheet method" for replacing pool water w/o draining the pool. I understand the basic concept, but I would like some clarification on the details.

Should I drain to just below skimmer level before putting on the plastic sheet and adding the fresh fill water?

I assume the sheet should be slightly larger than the pool surface area. Any specific suggestions on that?

Should I start adding chlorine as the new fill water goes in, or wait till the fill is complete? I expect the process will take several days, and obviously the filter won't be running.



Mod Squad
TFP Expert
Apr 4, 2007
SW Indiana
The sheet needs to be much larger than the pool. It not only has to cover the surface of the pool, it has to cover the walls without any gaps. The dimensions need to be the length of the pool plus the depth of the deep end plus the depth of the shallow end plus some extra to allow for the bottom not being a straight shot by the width of the pool plus two times the depth of the deep end. That's a minumum.


TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
Silver Spring, MD
If you use the sheet method the sheet needs to be significantly larger than the pool surface. Keep in mind that it will need to run from above water, down the sides of the pool, and across the bottom when you are getting near the end of the water replacement.

When I had to replace my water (CYA was over 300) I used continuous dilution. In this method you put water in one end of the pool and take it out from the other end of the pool, both at the same time. It helps if the intake and outtake are at significantly different water depths as well. This approach is much simpler but it takes longer and never totally replaces all of the water. If you are going for a 50-80% dilution it is fine, but if you need a 99% dilution it would be very inefficient.

The Mermaid Queen

TFP Expert
LifeTime Supporter
Mar 28, 2007
Northern KY
is your chemistry THAT bad that you need to change out all your water? could you do a few partial changes? I know you would end up using more water, as each subsequent change would then remove some of the new water, but I would think it would be a whole lot easier than the sheet method... besides the size of the sheeting as mentioned, you also have to be pumping the water out from under the sheet at the same (or similar) rate as you are adding it on the top, especially with an indoor pool -- you don't need a flood in your house!! :shock:
Jason's continuous dilution method also sounds easier than the sheet method...

As for adding chlorine, I don't know. First guess would be yes, if this will take several days. But if it will just be a day or 2, and your source water is chlorinated (ie tap water) you will have a little leeway, esp. since the sun will not be burning it off.



In The Industry
Mar 29, 2007
Knippa, Texas
It's not that bad. I have CC's (at 0.6) that are unlikely to go away and I'd like to get rid of the last of the Baqua Beast. Also have about 2-3 ppm copper which is a bit of a mystery--it is not present in my source water (maybe came from algicide used by previous owners). At this time I am just considering options. The partial dilution method could certainly work; my reservation about that is that it is inefficient. But it isn't critical that I replace the water. Even gradual dilution thru backwashing, splashout, etc., may be fine.

Actually the pool isn't in the house, it's in its own enclosure. So I don't have to worry about flooding the house. :) I figured I would do the filling while I was at home to monitor it, which is why it would likely take several days. My water pressure isn't that great, so maybe if I drained with the pump on low speed, the water running from the hose would be about the same rate. Or I might have to turn the pump on and off while adding the new water. I wouldn't want to try running the simultaneous drain-and-fill overnight until I'd monitored it closely for some hours.

Thank you all for the prompt replies! I think at most I will do a partial-dilution type replacement.


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