If you are lucky, your pool leak repair will be as easy as mine was. My leak, I was told by the leak-detection company is the most common type. The skimmer box is plastic, attached to the side of the pool. Shifting soil (California seasonal wet and dry) causes breakage.
My pool has a cover on tracks. No evaporation water loss. When the water level goes down, I know there is a leak. I paid a leak detection company 2-3 hundred dollars. I felt they did a good job with good equipment, but they couldn't find the leak. They told me most often the leak is around the skimmer box. They offered a fix they would guarantee. They would cut a 6" hole in the concrete aside the box, pump in an expanding foam which would form a seal surrounding the skimmer box pushing it against the pool side. They would replace the concrete and match the previous surface. The charge would be about $800. Result guaranteed.
How was I to know the leak was really at the skimmer box? They told me this: Add no water. Watch what level the water drops to. In about a month the water dropped to about two inches below the inflow to the skimmer box, but dropped no further. Ahha! They are correct! Is it odd about the water dropping two inches below the entrance to the box? No. Notice the pipe from the bottom of the box to the bottom of the pool.
Turning on the (suction) pumps, no water can enter the top of the skimmer because of the now low water level. Water level drops in the box until the bottom float goes down isolating the box. After that, water going to the pumps comes only from the pool bottom. Suction keeps the float down. The skimmer box remains empty of water.
You might notice the crack in the concrete around the skimmer lid. That's more evidence. If the lid wasn't so small I'd stick my head down in the box and look around. I got out my digital camera and took a picture.
The first thing I noticed was that the entrance to the skimmer box is a tunnel that is not level. It steps down. Even though the pool water was two inches below the skimmer entrance, water still gets in the box. Obviously, it comes up from below, from the pipe to the pool bottom. The platform in front of the phillips head screw was all wet. Above that screw, we see a whole lot of pool epoxy. That was put there by the young man from the leak-finding company. I paid him to do it rather than do it myself, because I figured he'd know better what to do. But he reached in from the outside, couldn't see what he was doing, and left some great prospects for leaks, especially that big wide slot below the screw.
Next I did something you should not do. You can skip this paragraph. I hope I undid the mistake I made then. I purchased some GE Silicon bathroom sealant. The label said in nice big letters, "water proof, flexible, shrink/crack proof". I didn't read the miniscule print that warned, "not for use in aquariums". I squirted it out the tube onto my fingers and attempted to apply it to the cracks. I couldn't! The goo stuck more to my fingers than to the wall of the skimmer box. Keep in mind that I couldn't see what I was doing until after I finished and then reached my camera down in the skimmer hole to take this picture. You can see what a lousy job I did.
Then I compounded my mistake testing my luck by adding water to the pool. Now turning on the pumps would not bring the water level down in the box to make the float valve go down. So I watched the water drop down 6" over the coarse of a month and tried again. I pulled out all the GE Silicon bathroom sealant that I could get a grip on. I replaced it with "Atlas Pool Putty". It is an epoxy that comes in two tubes. I have previously used it to attach tiles that come off. IMHO, it is wonderful stuff. I cut off about 1.5 inches, mixed, and did the best I could to push it into position. Upon completion I took the picture you see. I don't like the looks of it because I see a few potential leak locations. I reached in and tried tapping it there. I didn't take another picture.
Previously it took seventy 70 minutes to refill the pool. Today is April 10. We'll make the verdict by May 10, if not sooner.
Today is May 17. The pool water level is not visibly changed. Hooray! No need to add water. Good I fixed it before California's long drought summer.