Things You'll Need
Patching kit (available at pool stores)
Food color to detect leaks
How to Repair a Swimming Pool Bottom. A leaking swimming pool bottom is most likely going to happen with a vinyl-lined pool. It would take something pretty dramatic to cause a concrete pool to crack and leak at the bottom, whereas a vinyl pool liner over time will dry out, become brittle and be more susceptible to cracks. Unlike concrete pools, finding a leak in a vinyl-lined pool can be a difficult task.
Finding Out if You Have a Leak
Adding water constantly to your pool is a good indicator that you have a leak. A good rule of thumb is, if you need to add 2 inches or more a week, you probably have a leak.
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Determine if your leak is from an area other than your pool bottom. If it leaks only when the system is running, that likely indicates a plumbing leak. However if the pool leaks all the time, it's more likely to be a leak in the liner.
Walk around your pool and look for wet, mushy areas on the ground or areas where the grass is greener than surrounding areas. Either of these can be indicators of a leak.
Check inside your pool for any apparent cracks or sinkholes that might have been formed by water leaking out the bottom. Pay special attention to corners and the areas around steps where the liner may have been stretched tighter and be more likely to leak.
Look near the water line. If you live in a cold weather area, ice breaking up in the spring could cause a cut in the liner. An animal falling into the pool then clawing at the walls could also cause a cut in the liner.
Putting on a mask and snorkel may be necessary to find a leak right near the bottom. There may be small air bubbles coming from the leak. Or, since any water leaking will draw dirt in the water towards the hole, you should be able to see the leak.
Fixing the Leak
Patching a cut or split up to a couple of inches long or a small hole in a vinyl liner is relatively easy, once you've actually found them.
Buy a patching kit at your pool store. This kit will contain a piece of colored vinyl liner and plastic cement that will work underwater or above water.
Cut a patch to cover the hole, rough up the back of the patch with sandpaper, spread cement on the patch and place it over the hole (even if it is underwater).
Smooth out any air bubbles and leave the patching cement to set up.
Alternatively, a nitrile rubber-based sealant is available from some pool supply stores. With this product, you simply spread the sealant over small vinyl leaks without even using a patch. Even underwater it will set up and seal small slices in the liner. As an added bonus, it's often available in a blue color to match the liner itself.
Not sure how much water your pool is losing due to evaporation or whether it has a leak? Fill a bucket with water and mark the level. Mark the water level in your pool and leave the bucket beside the pool for a day. If you don't have a leak, the levels should have gone down the same amount. If the pool has gone down more than the bucket, you have a leak. Small leaks underwater can be difficult to detect. Using food color or the red phenol chemical in your water testing kit, squirt a little near where you suspect a hole. If there is a leak, you will see the colored water moving towards it. You can put together your own pool patching kit. For a dollar or two, most pool stores will sell remnants or pieces from pool liners they've installed. Vinyl patching cement is also readily available at the same store. That's it, for less than $5 you've got a pool liner repair kit (and you may even be able to match the pattern of your own pool liner).
If you find that you have a leak in your pool liner, don't drain the water from your pool. A vinyl pool liner is held in place by the water pressure in the pool. If you remove the water, the liner could slip.