Dick wrote:

It is my observation that filters are actually "strainers." They hold
the larger solids, but the water passing by is eroding the solids and
letting the stuff to return to the tank. Oh, woe is me, I predict
disaster due to negligence.

Well, I just discovered why one of my external filters' flow rate
dropped to almost nothing, even though it apparently looked clean. It
was driving me nuts. I hadn't cleaned the prefilter sponge for almost 8
months, but it looked pristine. I scrunched it in a bucket full of
water, just in case I was missing something. What did I then see at the
bottom of that bucket? Masses of white snail shells. Over the months,
tiny snails had got through the strainer then bred and died, finally
choking the sponge with their insoluble remains. The flow rate after
that was incredible. I'd forgotten how much water it could shift when it
was new.

Nikki