nut wrote:

amosf © Tim Fairchild wrote:

Correct. The bacteria adhere to all sorts of rubbish in the filter,
not just the media itself. All those particles that are rinsed off
and cloud the water have bacteria attached.

I have to admit, until now, when i've set up a new tank i've used 50% tank
water from water changes and 50% dechlorinated tap water.

So, although it's not harmful, using mature [dirty] water has no benefit
at all? From now on i should use 100% clean water?

I've also rinsed filter sponges in new tanks before too, in the hope of
speeding up the cycle... there seems to be mixed opinions on this but it
seems the general consensus is that it doesn't hurt so one may as well do
it.

However, now i have a few established tanks, i have the luxury of using
mature filter media in new tanks.

There are very little bacteria in the water column, so you might as well use
new water. There is some bacteria in the muck, but more on the media
itself, do throw the whole sponge it. Use some of the media out of an
established filter, or use a lot gravel from another tank. The more
bacteria you start with the quicker it can breed up.