Switching To External Filter

[Tropical_Dave]

so ive already got a tank up and running for a while, fully cycled etc, but im stepping up the filtration from an internal fluval u2 to an external fluval 304.
i was going to set up the 304 and have it running along side the u2 but im wondering how long would be a safe bet, i dont want to take the u2 out too soon and the 304 not be ready.
and would it be a good idea to squeeze the good stuff out of the u2 into the 304 before i set it up?

 

[Gruntle]

I'm not familiar with the Fluval filters, but can you move the media (sponge pads, bio ceramics etc) from one to the other? Or can you stuff the sponge into the canister somewhere?
 
Squeezing the "good stuff" is not really possible as (I think) the bacteria have a good hold on the sponge. You might get some bacteria out, but I'd think the vast majority would stay in the sponge.

 

[Ninjouzata]

I would move the filter media from the old filter into the new one

 

[Gruntle]

Bacteria will colonise where they get the best nutrition, which in many cases would equate to water flow (but not certainly). You would likely eventually end up with bacteria in the new filter, but depending on how well the existing one functions, the bacteria may continue where they currently are. There's absolutely no way to tell if or when the new filter would have a sufficient quantity of bacteria to safely remove the old one. Bearing in mind that bacteria will only reproduce enough to take care of the waste (ammonia and nitrite) so an extra filter won't automatically mean more bacteria. If they colonise the new filter, the bacteria in the old filter will reduce, but again there's no practical way of telling if and when that has happened.
 
Best bet is to move the media in which case you'll be likely to move as much of the bacteria as possible and avoid a mini-cycle.