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I have one of those HOB filters with the replacement cartridges containing activated carbon. I recently took out the cartridge and replaced it with a big piece of filter foam instead because I read that activated carbon removes ferts from the water column.
Ever since then my tank has become somewhat cloudy and it looks like a bacterial bloom.
Is my tank going through a mini-cycle? I've never replaced the cartridges before and I'm not sure if thats the issue or something I haven't accounted for.
It's a 20 gal long with high light and injected co2.
 

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Nitrifying bacteria will colonize the activated carbon, but it takes a while to build up the population.
When you leave the cartridge in for a month or less, and keep replacing it, more bacteria grows elsewhere, and not as much on the activated carbon.
But if you leave the cartridge in the system then the bacteria live on the carbon. Then, after several months you remove these bacteria the bacteria that remain in the system on the substrate, decor and elsewhere is not enough to deal with the ammonia produced by the fish.
It is rare that cloudy water results from the growth of the nitrifying bacteria, they are slow growing.

Check the ammonia levels. Rising ammonia can cause cloudy water.
Heterotrophic bacteria can reproduce fast enough to cause cloudy water.

Activated carbon has some affinity for chelated minerals. But, since AC gets filled up pretty fast (few days, maybe a week) the plants are not actually going to have any problems with lack of fertilizers. A couple of days without a mineral is not a problem, and there may be some reserve in the substrate, anyway, so no lack at all.

I have replaced all the cartidges in HOB filters with a stack of media that is more like the set up found in the Aquaclear HOBs. This is a very efficient way to remove debris from the system compared to a thin cartridge.
I have not run AC in any tank for many years. I have some on hand in case of emergency (a toxin in the tank, or to remove medications).
 

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Most keep the carbon pads for future use when they need to do meds in the tank
for example. Using the carbon after the meds have done their thing may help to
remove the meds from the tank although I think that water changes likely do more.
Sounds like your filter has a three part system. Bio-media and two pads before it.
Two sponges and the bio-media pad/sack etc. The center pad will usually also
contain a fairly high percent of the beneficial bacteria. So when you removed it
you likely did start a mini-cycle. Whatever you replaced it with will now also act
as part of the bio-media.
If your in the habit of draining the water change water into a 5g bucket you can rinse the two sponges before the bio-media in the water in the 5g bucket without causing
a mini-cycle. Just shake them enough to get the dirt out that they have collected.
The majority of any "dirt" should be in the first sponge most of the time.
 
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