Not familiar with the construction of your pool. However, this is typically done by boring a 3/4" line into the side of the pool below the water level and out under the deck or into the deck. Then a pool filler reservoir is placed there. This is the same as a skimmer reservoir without the flat inlet in the front. The level of the water is controlled by the device I specified previously which is connected to the pressure line piped into this reservoir. As the level drops in the pool the level drops in the reservoir and the reservoir is refilled to increase the level of the pool. This also has the added benefit of not being affected by splashing or rough water caused by windy days and the like.
In a retrofit this will require boring on the plaster below the beam and finding a location anywhere around the perimeter of the pool for the reservoir. BTW I would avoid those that look like they belong in a toilet tank as these are typically made of plastic which is susceptible to freeze and temperature change. While you are at it a good addition is an overflow for when it rains so that the water level never goes to the coping. Water can seep under the deck just below the coping causing damage to your pool deck.
However, there is another solution that might also work for you if you already have a manual filler line piped into your pool.http://ezautofill.com/
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