as long as the water in the tank has LESS fixer & contaminents than the prints water, the wash water
will leech the other materials out of the prints ( diffusion ).
once the tank's water has the same amount of fixer &c as the prints, it has achieved equilibrium ...
and no more of the contaminants will leech out of the prints into the wash water.
folks often times just fill these washers ( or even a large tray or bucket ) with water. let the prints SOAK.
contaminants leech out of the prints, they drain some of the water( or if it is a tray or bucket dump and re-fill ), fill it again, soak some more
leech more contaminants out &c. it uses less water and does similar things ( washes the prints ).
this same wash method is used to remove fix&c from film, but it is easier because the film base isn't a sponge ( like paper ) to absorb the chemistry.
and it is for this reason RC paper was invented.
https://en.wikipedia.org/wiki/Diffusion