Have you read Pool School?
I would advise you to:
Order your own test kit
Shock your pool with liquid chlorine
Stop using tabs (possibly)
I would wait to change the sand, it may also be that you are backwashing "too much" - dirty sand filters better. You can try adding a little DE to the sand to improve clarity - instructions are in pool school.
Tablets are acidic - the lower PH/TA and they raise CYA.
If your CYA is too high, your chlorine becomes ineffective - the higher the CYA the higher the FC must be - refer to the chart in my sig.
So without knowing what your CYA and FC, CC levels are, we are guessing at this point, but I suspect you need to lower CYA and shock the pool.
You need your own test kit so you have total control of your pool. Instructions on how to shock your pool are in Pool School - along with an article that compares the three test kits we recommend (which you'll likely only find online, so order today! LOL)
If your CYA is higher than 70, you will need to do a partial drain/refill to lower the level to a manageable number, then proceed with shocking.
If your CYA is 50 or higher stop using tabs today.
Hope this helps.