Firstly, thank you for reading about the nitrogen cycle before getting fish! That's a +1 straight away.
Biofilter is a bit of a funny term, really, because all filters technically become biofilters as long as they have media that would allow beneficial bacteria to grow on it. A lot of your beneficial bacteria will also live in your substrate, if you have any, so you'll gain some that way, too.
Because this is quite a big subject, I'm going to break this down into a few sections.
What do you actually need for beneficial bacteria?
It seems like it doesn't take anything particularly elaborate for the bacteria to live on
Exactly correct. Indeed, many people just use pot scrubbers because they're cheap and readily available.
This is because all the bacteria really need is a surface to hold onto and enough oxygen to do their work. Some people argue that in order to cultivate the most beneficial bacteria you can in the limited space you have is to get something that has a reasonable surface area like BioHome Media because it is so porous. There doesn't seem to be a huge amount of readily available information out there about the merits of this type of media, but I've got some and it seems to work slightly better than, for example, ceramic rings.
Activated carbon is used to absorb particles from the water. It is primarily used to keep water clear, remove odours and generally remove dissolved organics in the water.
Is it that necessary? Well, I've often used it to remove tannins from water (brown discolouration that normally occurs because of bog wood) and it's been very effective. It's also great for absorbing old medicines from the water. Necessary? No, probably not. It's pretty inexpensive if you buy it in bulk and then you can just put it into a filter bag and you're done. Once the carbon is completely saturated with organics, you'll want to swap it out for fresh carbon.
These normally just have bio media (like k1 micro) inside them and some cheap foam to grab the large particles. These allow you to just swap out the cartridges every now and again. They're pretty common in small aquariums because they're compact. These will also harbour beneficial bacteria because as long as they can hold onto something, they'll be able to detoxify the ammonia.
What else do I need beyond the beneficial bacteria from a filter?
A few things spring to mind:
- Easy to maintain (I've had some cheap filters, but they're always a pain to clean and break more easily).
- Enough filter sponges (So they'll take out any bigger particles that might clog up your bio media).
- A high enough flow rate (You need to turn your water over a couple of times per hour, so check out the flow rate).
Finally, can you have more than one filter?
Short answer: Yes.
The long answer is that you should try and calculate your bioload based on the fish you'll keep and your maintenance schedule (water changes, filter cleanings, etc.) and tank size. Then you can match the bioload to a filter. In reality, this is a pretty complex calculation based around the amount of ammonia bacteria can process and a guess at how much bacteria you've got.
If you've got a relatively normal bio load in the tank, a single filter rated at the size of your aquarium (e.g. filter for 100 litres on an 80 litre tank) will be perfectly adequate. You could happily have another filter in there to further increase your filtering capabilities and flow rate in the tank.
If your filter is too big, like a pump rated for a 500 litre tank in a 30 litre tank, you'll might run into problems because the flow rate could be too high for the fish and the tank.
The key thing to remember is that the beneficial bacteria are what converts the ammonia to nitrite and then into nitrate. If you have enough beneficial bacteria, you won't have ammonia in your system anyway. Another filter will only add the ability to cultivate more bacteria and process more ammonia.