We've sold dozens of
Hagen Fluval Underwater Filters.
1plus, 2plus, 3plus, & 4plus.
I love these filters.
They have 1 instruction.
Step one: Plug in.
The problem is
I have sold almost NO replacement foams.
(Not that we're only here to sell stuff.)
I'm worried that my customers
are going to have zero flow soon in their tanks.
Most foam filters can be rinsed many times in cool water
but the Fluval Plus foams only seem to stand up to 2 or 3 rinses
before they flatten out.
So anyway,
if you have a Fluval Plus internal filter (or any filter, for that matter)
rinse or replace the sponge at least every 6 weeks
to insure that you have maximum flow.