Change your hydrostatic valve every 6–8 years
The little plug at the bottom of your pool causes leaks all the time — and it's one of the cheapest things to fix.
The hydrostatic valve is the plug at the bottom of your pool. It comes either as a screw-in 40mm or 50mm plug, or as a larger valve about the size of a dinner plate.
The screw-in plugs should be changed every 6 to 8 years — they perish, and when they do, they cause leaks all the time. Changing a 50mm plug costs about $150, and while I'm there I'll check the rest of the pool for leaks too.
The pool needs to be in swimming condition to do the job, and summer is the best time. If you can't remember when yours was last changed, it's probably due.