Welcome to AA!
I understand how overwhelming the information can be getting started!
I think that it really helps to "paint a picture" of the ecosystem you are building - and for me, it was the key to my *eventual* great setups.
Here is a super-simplified overview that helped my friend - I was emailing her for days, ha ha!
She had just realized that she had gotten a very large fish (at baby size) and was thinking over what she wanted to do...
Like any environment, an aquarium has a cycle of life. The neat thing is that this is how that little world stays clean.
Waste from uneaten food, poop, dead plant matter, etc. creates ammonia - this is the most toxic thing you test your water for.
This is food for bacteria that can live on your filter media and tank surfaces. They change it to nitrites - a little less toxic to fish.
The nitrite feeds a second type of bacteria and that gives you nitrates. These are toxic too, but not enough to hurt thanks to partial water changes.
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Ok so now you just need to create the drainage of dirty water and renewal that naturally occurs (through runoff and rainfall).
Removing water, (with a hose, for example), is your drainage. Adding new water is your rainfall. This gets your nitrates down. Plants do that too, if you have any.
You should take your filter stuff and wash it out in your drainage water once every couple months. Be gentle so the bacteria can hang on and only replace it when it falls apart.
Carbon media for filters is different! That removes chemicals that come from the bottled liquids you use or city water components. Many people do not use this stuff at all because they do not have anything like that in their water. If you are giving liquid medicine to your fish, you have to pull out the carbon until you are done or it will just yank it back out if your water! It needs replaced to be active every couple months.
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So, when you plan who gets to live in your tank, you basically figure out the swim room needs of the fish and it's bioload - the output it creates for your cycle.
Some fish, like neon tetra, have tiny bioloads so if you have the swim space you can do more than 1 inch of these per gallon.
Some fish, like every pleco or catfish, have huge bioloads. These types do not process food the way we do so they create way more waste. Mollies are also high bioload.
The reason to test your water is to make sure the bacteria and water changes are keeping up!
Thus, your scheduled changes and filtration system affect what you can keep.