On an above-ground pool, the skimmer fitting is secured to the side of the pool with a face plate that is sealed with a gasket. This skimmer outlet can sometimes develop leaks, either because the skimmer face plate has become loose, or because the gasket is old and has developed cracks. If you notice your water level dropping for no apparent reason, it's very likely the water is leaking around the skimmer face plate in the side of the pool. This can waste quite a lot of water, and it also prevents the skimmer from properly doing its job to remove debris from the water. And because the debris then sinks to the bottom of the pool, this means you will need to do more frequent vacuuming or hand cleaning.
Fortunately, repairing skimmer leaks is simple. Buy a new skimmer gasket that will fit your pool skimmer. Many are "universal" gaskets that fit most above-ground pools, but always check to make sure the replacement will fit your pool model.
Check for Loose Screw Fittings
It's possible that the leak is caused simply because the fittings around the skimmer face plate have loosened, allowing water to escape around the gasket. If you find loose screws, tighten them and see if this corrects the problem.
Drain Down the Pool
If tightening the mounting screws doesn't solve the problem, then it's likely the gasket around the skimmer fitting needs to be replaced. Start by draining the pool to an inch or two below the bottom of the skimmer face plate.
Remove the Skimmer Face Plate
Remove the face plate from the inside of the pool, turning the nuts in the back of the unit with pliers or a socket wrench while holding the screws in the front of the face plate stationary with a screwdriver. These screws hold the skimmer unit in place and prevent it from falling to the ground and cracking.
Remove the Skimmer and Gasket
Remove the skimmer unit from the back of the pool and the skimmer face plate from the inside of the pool, being careful not to drop the screws into the pool. Peel off old caulking carefully and remove the skimmer gasket. Make sure to scrape off any gasket material stuck to the side of the pool.
Install a New Gasket
Coat one side of both pieces of the new skimmer gasket with silicone caulk, and press one piece on the outside of the pool wall and the other on the inside of the pool wall. The gasket on the inside of the pool wall will be between the pool wall and the pool liner. Coat the other side of each gasket with silicone caulk. Coat the back of the skimmer face plate with silicone caulk, and push it back into position from the inside of the pool.
Put the screws back through the face plate, aligning them with the existing holes in the liner.
Reinstall the Skimmer
Lift the skimmer unit back into position on the outside of the pool and align the screws with the holes in the skimmer unit. Hold the screws through the face plate stationary with a screwdriver, and tighten the nuts on the back of the unit with pliers or a socket wrench. Do not overtighten or the face plate or skimmer unit may crack.
Fill any visible cracks in the skimmer unit's housing with silicone caulk on both sides of the unit. Inspect the plumbing around the base of the skimmer unit. If the plumbing is made of flexible pipe and attached with a hose clamp, tighten the clamp slightly. Do not overtighten or the connector on the base of the skimmer unit may crack. Wait 24 hours before refilling the pool, and check for leaks.