Saltwater battery? Detailed explanation

Hi all,

I've recently been browsing all I can on Saltwater cells (well technically galvanic cells) Yet I cant seem to find an explanation as to why the saltwater solution needs to be changed in the cell? From what I've gathered through google searching the salt in the solution is not used up but I can't figure out why the cell begins to fade after X amount of time (cleaning the anode and cathode does not seem to do anything) 

Any help on the subject would be vastly appreciated! Google does a poor job of finding detailed results and most "Science" pages give the general principle of the cells operation but no details of why the solution needs to be replaced.

Thanks!

(here is an example video, the author says to change the solution after a certain time period but doesn't know why)