In article ,
says...
On Aquaclear hang-on-the-back filters, I've successfully used foam as a
pre-filter, placed/ rubber banded over the intake. I've used foam
sleeves sold for use in Fluval in-the-tank filters, and also carved a
foam sleeve out of filter foam.

In fact, if you take a block of foam and cut a slit in it, you can push
it up ove the Aquaclear input and not need the rubber band. At least
with smaller fish - large goldfish might require the rubber band.

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