I'm not sure I would be too alarmed about your foundation yet, but it's good that you're aware. Sounds like you have a hole in your liner that is allowing water through to your pool's concrete shell, which can hold water fairly well. If the water level continues to drop every day, then I would be deeply concerned (but if your pool was working fine before, I doubt it will). Try looking for rips in the liner along the water line. It could be a small leak, which could easily be repaired with a vinyl liner patch kit; they are inexpensive and should be available at your local pool supply store. If you can't find the leak (or if the patch doesn't hold) you may need a new liner. Liners are designed to last approx. 10 years, so if you are near that age you likely need a new one. New liners also cost $3,000-5,000 in my area which is comparable to the cost of filling in a pool. I'd try talking to a few of your local pool supply stores about which companies in your area can provide this service. Bids can vary widely for pool-related services so be sure to get a number of them. Good Luck