Has anyone weighed the bottles when received? Lots of complaints about 1/2 empty bottles. Sold by weight.

I was curious about this too so I weighed an unopened container and it weighed 2 pounds. The reason for the taller bottle must be marketing related. Personally I feel it makes it easier to control how much you pour out of it. All that being said I have been using it in my hot tub for quite some time now along with regular doses of chlorine and my tub is always crystal clear. I have no complaints with this product.

Why doesn't it say shock on the bottle ?

Shock simply means it works fast. Like 15mins.

I read everywhere to "shock once per week"... Why does this bottle say "after every use" ?

Shock after each use and if not in use, shock once a week.

My container came 3/4th filled and without a seal. Should there be a seal?

Yes, I have. Just got a bottle and top ajar and signs of crystals in shipping box and 1/3 of product is missing. They will not allow a return or replacement. Shame on seller and Amazon.