How Long...

[joecoral]

would it take a completely new and sterile filter to seed and mature if run on a mature tank alongside an already mature filter?

 

[KnuckleHead]

I would give it a minimum of a months time to establish a good colony. Remember the colony will only grow as large as the bio load you have in the tank. While both filters are running, the original filter colony will eventually diminish while bacteria colonize the other until you reach a sort of equilibrium. This is something that you do not want since you'll be removing the secondary filter and leaving the main with basically half of the colony it used to have. You need to minimize this by getting a secondary colony just started and go from there on your other tank. Ugh I hope that made sense.....

 

[Shrimper]

Not something i've done but would guess at least a month to be certain there is enough bacteria in the new filter. The bacteria in the filter will only support the amount of ammonia being produced by the fish in the tank, by running the new filter in the tank all you will be doing is giving the existing bacteria a new place to live and you'll have to wait for them to move across.

One thing about the bacteria in the filter is so long as it has enough food the population can increase by 100% in 24 Hours. Because of this its probably easier to do what i and transfer some of the existing media to the new filter and add a few fish straight away. I used this method when i set up my big tank by moving my existing corys and some of the existing media sponge cut in half to fit in the new filter. If you move the media at the same time as you add the fish you shouldn't have any problems.

You could do a hybrid of the two methods if you really wanted and run the new filter with some existing media in it for a few days in the existing tank but its probably not worth it.