You've been "pool-stored". You need to go to WalMart or the pool store, and get a drop-based kit (WalMart sells a 5-way for around $15). Run your own tests for Cl, pH, Alk, Calcium, and CYA, and post those numbers. With numbers to work with (not "a little low"), we can help you fix the problem.
I think the problem probably is that you have little or no stabilizer in your water, so the sun is consuming what chlorine you put in. With this condition,it doesn't matter how much you add, and in what form, it's still going to be gone by the end of the day. LIkewise, if you've got something blooming in the pool, it's going to consume large amounts.
Post up some good numbers, and we can steer you in the right direction. Also, tell us if the shock you've been using is dichlor or cal-hypo.
Janet
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