Penn Plax Cascade 300 GPH Hang On Aquarium Filter Cartridges with Activated Carbon, 3 Pack

Will this fit the cascade 300 hang on filter? here is the link https://www.amazon.com/gp/product/b0002dj82g/ref=crt_ewc_img_dp_2?ie=utf8&psc=1&smid=a2

I bought a nine pack and only received three. Can anyone help?

I have a 65 gallon aquarium for 2 sliders. How often should I change this filter?

How do i know when the filters are spent? I had an algae bloom and changed mine out probably too quickly to try control it.