I have no clue where to begin so I will start with the design of my in-ground vinyl pool liner. The dimensions are: 36' L x 16' W x 5' 9" Deep at Center and 3' Deep at both ends; sports/volleyball pool. I had it for 10 years and it sat empty about 5 years. I had to dig out 10 gallons of dirt out of the bottom of the pool from one part of the wall which caved in. The bottom was dirt but now filled with 2" concrete. The retaining walls are made from plastic or fiberglass. I have two returns, had one but now two main drains, and one skimmer. The pump is 1.5hp with a Hayward 1750 cartridge filter; the pipes are 1.5" PVC. The steps are the fiberglass inserts with liner cut and sealed against steps.
With that said, I recently replaced the liner; filled it up; and now checking for leaks. After the water reached the arrows on the skimmer a week ago I have been watching the water level drop about 1" to 2" per day. In the past week the water level dropped from arrows to below the bottom of the skimmer faceplate (non-wide mouth) about 6". Just last night the water level for the whole day was right at the screws of the skimmer faceplate; this morning it was at the bottom of the faceplate it dropped 1/2" overnight.
My question: could the pool be settling or could it be leaking? While the water was at the skimmer faceplate screws I used red food coloring to test for leaks. I dropped food color by all cuts in the liner: pool light, both returns, steps, and skimmer. Of course the skimmer could not be tested but I retightened the screws while the water level was below and now filling it up again to test. I watched the food color spread through the water and cleared. Some of the food color lingered at the steps but not in the location where the seals are. I will test again when the water is at arrows.
I checked the water level inside the skimmer and it matches the pool water level: I don't have the skimmer float valve which I plan to buy soon.
Before replacing liner I noticed certain concrete slabs around the topside of the pool has shifted towards the pool meaning it was on the brink of cave-in. I have been checking the slabs for settling as well; haven't notice any movement but I am not taking exact measurements though which I should.
I have no clue what it could be: the pump has been turned on for a few minutes to check operation but off the rest of the time. My ideas/fears: the issue could be the main drain hose to skimmer because it was my friend who glued it not me or the seals around: skimmer, returns, drains, or light. I am leaning more towards the ground around the pool is settling back to original location but would like other opinions. The food coloring showed no sign of leak. I checked all the walls for floating liner but tight against retaining walls. After the concrete was done: the main drains sat with water for over a week after a flash flood adding about 1' of water; when I installed the liner I had to suck out the water from the drains.
If I can remember anything else I will update post or reply.
Thank you in advance.