The "cotton" filler, or "fluval", as it is known as here, does not wick
juice to the atty bridge as well as the blue foam. Fluval holds on to
juice too well, and so you don't get all the juice from the cart before you have to add more. Blue foam tends to give up all its juice better.
I tried a bunch of cart fillers, and the foam was the best of what I was able to get working well. Modding carts, however, often is frustrating in the beginning, as until the design is correct, the cart will sometimes leak...well, actually, carts tend to leak no matter what you do, but I had less issues with foam than other mods. It doesn't have to be Blue Foam (Marineland brand). I use Aquaclear aquarium foam, which is sort of off-white, but it is the same course sponge-polymer material. Works great. You are right about the stray thread issue, but it is really all about the balance between holding liquid and wicking it to the bridge.
