with the Hydra unit, you have to go through the large hole in the center... you just cut it into small enough chunks and pull it out piece by piece...
with an Oasis you can pull the top off of the container.. just slip a small screwdriver into the seal and turn it near the 4 corners.. the top will pop right off...
Havent looked at a Humi-Care close enough to know how to get at the foam on that one.. but Im guessing one of the 2 methods above would work...