Without stabilizer, you can lose 90% of the chlorine in your pool (regardless of how much is there) in 2 hours of full sun. There are lots of variables here -- how clear is your water, how "full" is the sun, how deep is the pool -- but you get the idea, I hope.

Also, if you HAD stabilizer in your pool last year and didn't, when you opened this year, and did NOT drain your pool, you may be WORSE than unstabilized: you may have no stabilizer AND have ammonia left over which bacteria 'pooped' out after eating your stabilizer.

Best way to check: add chlorine late in the day; test levels an hour later and then again in the morning within 2 hours of sunrise. If there's a big difference, you have some sort of goo in the pool that's using up your chlorine.

In general: lose chlorine at night = goo; lose chlorine during day = many things but often, no/low stabilizer.

Ben