Carbon Cartridge replacement without removing bacteria
 

Hey,
I've got a basic undergravel filter with carbon cartridges that the seller advisers i replace every 4 weeks. I have a 1 black moor so it's not exactly alot pressure on the filter to clean. I was wondering if i actually should replace them this often because they will soon harbour helpful bacteria that converts nitrites into nitrates? If i do replace the cartridges, how can i transfer the 'good' bacteria from the cartridge to the rest of the tank? Sorry this bacteria thing is new to me.
Cheers
HM :-)

i have no experience with undergravel filters... but in your post you referred to "cartridges," so I am assuming there is more than one cartridge in the filter? You can just change them one at a time instead of all at once. That way you will always have some beneficial bacteria in the filter. Like... say if there are two cartridges, change one, and then change the other one two weeks later and keep rotating through them like that.

No it has only 1 cartridge, I was just wondering if the bacteria will actually spread to the gravel?

You could fit a clean cartridge in the filter beside the old one, and every four weeks throw away the oldest one...

Then again, since you have an undergravel filter, the gravel is the main bacterial medium. You should be fine tossing the old one. (An UG filters main advantage is the high levels of bacterial filtration)

Personally, I don't use carbon... It does keep the water clear, but if you change the water regularly it's unneccesary. (And harmful if you have plants. Some floating plants like duckweed, azolla, and amazon frogbit would do the same thing, and would be tasty for the moor.)

No it is only single cartridge, I was just wondering if the bacteria will actually spread to the gravel or anywhere else?

Yes, it will. The amount in the cartridge is really insignificant when compared to the entire inside of the aquarium. Go ahead and change it, you won't be doing any harm.