You didn't say what your tub is made of. And belive me, depending on what it's made of (or better yet, what's it FINISHED with), you may be using the wrong cleaner. Also, since you clean it every day, you needn't use such a harsh cleanser. If I could bring myself to clean my tub every day, I'd use a liquid cleanser with a non-abrasive sponge. Comet is more for those folks that clean their baaaaathrooms every week or so. (Ooooh, not me!)
In all seriousness, that's the best I can do. If you can find out what the finish is on your tub/shower, go to a Bed and Bath store / home-center and ash what you should use in your 'situation'. Better yet, if you know the mfgr., contact them!!! My best to ya and hope this helps.