how to install new filters in rodi unit

[buddyreefer]

hi

i just changed out the prefliter membrane and carbon membrane in my 3 stage rodi unit, but i'm not sure if its working properly, it seems like water just fills up, i don't understand how the water goes thru them? can anyone explain the mechanics of the filters, i.e its very easy for me to see how the 2lilfishies phosban reactors work as the water goes down and up thru the material but i don't see that happenning with the prefilter membrane as it just fills up from the outside?? sorry for the rambling post just hard to put into words.

thanks