MarineLandPenguin Refillable Media Cartridge, Fits 200 And 350 Power Filters (PA10093)

What do you put inside these?

I'll be using two of these, and two disposable .Does it matter which is in front and which is in back? Is this overkill?

How many come in a pack?

Would polyfill be ok in these? I have a 350 and plan on using the first two with polyfill and the back two as my biological/chemical filtration.