Hello; I transfered the fish from QT to their home tank. I pulled the sponge filter and gave it a few good squeezes in a bucket of old tank water. The foam sponge material was still in good shape. It was intact and not structurally weak. I have had some fish in the QT for several weeks at a time with no apparent ill effects. I have one long term resident of several months who is doing well. My take on this test thus far is that this foam material works well as sponge filter material.
I plan to try to fit the material in one of my HOB filters. It may be too dense to flow well enough but is worth a try.