Theoretically, if chlorine is kept at the proper level, you never need to shock a pool. However, if you chlorinate exclusively with tabs, which I assume are trichlor, you are continuing to add cya which, over time build up to levels which require higher chlorine to keep algae at bay. I suspect this is what is happening in your pool. Even though your tester shows yellow, your cya level may be so high that you should actually be keeping your chlorine much higher than you are. We actually need a complete set of current water testing numbers to be able to help. Repost with numbers and somebody here can better advise you. We need FC, TC, Ph, alk, cya. Also tell us what type of pool and the volume.