David F wrote:
I have a tank with an UGF and an internal sponge filter with an air-hose
venturi.(for redundancy, the tank is not overstocked)

Sometimes I like to turn off the sponge filter to have quieter and
stiller water during the day.

It occurred to me that if either internal filter is turned off, then
harmful anaerobic bacteria may grow in the stagnant filter media (gravel
or sponge) at the expense of the beneficial nitrogen-cycling bacteria.

Is this correct and do you think that it is ever safe to turn off a
redundant(extra) internal filter?

The problem IIRC is more with turning off external filters for a long
period of time (hours) and that you could get away with turning off your
internal for a short period of time. The difference being that one is
actually in the water but with an external being turned off the water in
the chamber can stagnate....I've actually had my externals off for
around 10 hours during a power outage and they have not been a problem
when restarting (but I wouldn't recommend that)....the main PITA with
them is getting the older ones restarted (have to manually prime them)....

As for whether it is safe to ever turn one off - of course it
is.....afterall you need to turn them off to do maintenance work.....

So I would reckon you should be OK if you only turn it off for a short
period of time - and remember to turn it back on again....if you really
want to err on the cautious side empty the cannister and rinse the
sponges in tank water before turning it back on...

Gill