don't overturn that tab. if you go too far it won't come out. line the arrow up square with the "open" label, and take your time pulling it off. it'll be pretty hard to do after it has been ran.
what i do is turn off the intake from the tank, turn off the outflow, move the filter to the shower (saves getting my tank stand wet) then open both taps to let the pressure out. then try the tab and it should be easier.
once its off do what you have to do with the filter ie clean the foam etc, reassemble, fill with water thru the tab til its full, reattach the hoses/taps, then crank it over. should be good after that.