Can your pool leak that much through those tiny cracks? Absolutely. I've seen a hole no bigger than a pinhead because of improperly applied pebble have a pool lose 3"-4" a week. If it is plaster cracks it can be an easy fix. The best thing to do is to dye test the cracks to see if they are leaking. If they are drain the pool down and cut out the cracks and remove the plaster around the cracks to see if it goes into the pool shell. If the shell has no cracks than you can patch the plaster. If the shell is cracked you may have some structural issues. Do not try to epoxy the pool shell if it is cracked. That's a band aid on the problem. You will have to get a soils engineer out to look at the soil and then core test the pool to see if proper strength for the shell was met. You may have to demo out the pool shell in that crack area to see the extent of the damage and to see if the re-bar is rusted. Hope this helps answer your question.

I suppose the interior finish has to be plaster though it does not look like it.

This is a dark gray pool. Some call it black. The surface looks like it is made up of lots of small dark gray granules. Have no pics, as it is 200 mi away! maybe later. Any ideas what that finish might be?

I understand that they could be just plaster cracks (if that's what it is) or may be all the way through the gunite. I'm thinking it's likely to be all the way through, because of the continuous lost water. but maybe not.

Down the road, if I see deeper and wider cracks, I will do what I have to do regarding the pool structure.

The first thing I plan to try is to stop the pool from leaking, and right now I'm trying to get ideas on that!!

Thanks again for any tips on how to stop a leak..

It sounds like it's a quartz mix. What I suggested is the way to stop leaks. I know it's not a quick fix but it's the right fix.

Is a quartz mix something mixed into the plaster?

Curious if other folks reading this forum have ideas on near term leak stopping.

Some use a type of super caulk.

Ideas on the near term fix are much appreciated!

etbrown4,
A temporary fix would be to drain the pool below the cracks and use similar color grout(non-sanded) to fill the cracks and see if it stops the leak.
The description of your cracks leads me to believe that they are not stuctural and your leak my be some where else. Turn the equipment off and let the water retreat to the level of the leak and then check everything at that level. Good luck.

These are true hairline cracks. Not nearly enough crack to grout as they are.
I see 3 options:
1. Top coat them with something like Vulkem 116 calk
2. Cut out the crack with a diamond blade and fill with matching plaster.
3. Make the guess that none are leakers and fill the pool.

Then dye test them.

Suggestions welcome

Have you checked the equipment side? Alot of times the mutiport spoke gasket will leak out of the backwash line.Below is a link for the people that do leak detection for a living and are very good at what they do...

Here is a link that might be useful: American Leak Detection

Thanks for the post.

That's a good point, and might still bear further study.

We bypassed the multiport valve and have applied 20psi direct pressure to the skimmer and return lines. There are no leaks in either.

It is true the multi[port can leak and send water to places you don't want it to go, like the filter or out to waste etc.

We'll take a look at that as we'll still have to find about a gallon a minute leaking somewhere. Since the below ground pipes have been tested, it would need to show up above ground, I think. That is unless we're back to the pool structure itself.

Thanks for the tip..

A Likely Solution?

I believe I have found and stopped the leak. At least for now, and seemingly for the most part. I'm posting this in thanks for those who contributed above, and hopefully to help others who may encounter a problem like mine in the future!

I was loosing 4" overnight and now, it's maybe 1/4" or less overnight. (Temps here are 40-50 so evap should be at a minimum). I realize I still may have some small leaks.

I spoke at length with a guy who plasters new pools professionally, asking about those hairline cracks. He said that since they were above the winterized water level, he suspected that they were due to drying out of the plaster with the water level lowered. He finds that the hairline cracks like mine usually disappear once the water level is raised, due to adding moisture back to the plaster. He said that in 20 years of plastering pools, he had only found one pool materially leaking through this kind of hairline crack.

So just to be cautious, I did apply a skim coat of silicone to them before refilling, though they were not my main suspects!

I got on a mechanic's creeper and spent several hours examining the tile line around this 40' pool. I found about 8-10 openings in the grout between those tiles at various places from 1/4" up to 2".I cut out the bad grout and replaced it with silicone for a temporary repair. I know that matching grout might have been better - I just didn't have any available on short notice.

I also spent a fair amount of time in and around the pool side openings to the skimmers. There was some obvious missing grout there, and I did carefully seal the vertical and horizontal joints between the skimmer box and the plaster. (On my pool, that vertical joint was up inside the skimmer, maybe 4" from the face of the pool wall) I let that cure about 8 hours and filled the pool.

So far it's 95% better than before! Now that I can finally keep the water level up to the skimmers, I can finally run the pump . Once we get a clear pool, I will dye test everything around the tile line, the skimmer box, and those pesky hairline cracks, just to make sure.

Interestingly, in posting this on this and a few other forums, I don't recall anyone suggesting a look at the tile line. Thankfully, numerous folks did suggest having a look at the interface between the skimmers and the structure.

It's a great relief so far, though I know I will have to keep an eye on things, and maybe later cut out the silicone and replace it with grout!