Basically, if you want to stock your tank heavily, you want to have 10x the filtration of the tank. So for demonstration purposes, we'll use 100 gallon tank. You'd have to have 1000 gallons per hour of filtration in there. I like to use a variety of ways to accomplish this. Say you have a canister filter that does 500GPH(gallons per hour) Then you add a power head, with sponge attachment. Which is rated for 300GPH. You're now up to 800GPH. Add in a Hang on the back filter of 300 GPH and you've hit 1100 GPH.
Now that would be with all the factory specs, with optimal conditions, which most fish keepers probably don't have. There are other things to consider/ways to optimize your your setup too. Put sponge filters on the intakes of your filtration. Get a round piece of sponge, get a hole cut in the center, and put it on the intake. This forces the water through it. It's gonna do two things for you. A it's gonna filter out big debris, and two it's gonna be perfect for growing the good bacteria you want. This bacteria will further filter the water as it goes through it.
Water Circulation is next. If your tank has dead spots, and fish waste is there, it will never make it to your intake filter to be processed and cleaned. Thats how the power heads with sponge filters on em help. It's a two for one, water circulation and free filtration!
Next is gas exchange. Without proper water surface agitation, a film will often develop. From the oils from food, your hand, waste etc. Might look like an oil slick. This prevents the Co2 in the water from getting released through the top. The use of an air stone doesn't really physically air into the water. Rather instead it agitates the water surface allowing the exchange of Co2 out, oxygen back in! When you do use a air pump, use it in conjunction with a sponge filter, the air going from the bottom of this filter, to the top of the tube which creates an upward force/siphon. This will draw the water through the bacteria filled sponge, and collect more large debris for ya.
Different fish put out different amounts of waste. If you want to overstock the tank, say due to aggression issues. You have to up the filtration.
Also don't forget about water changes. This is the cheapest way to keep your water parameters up, and reduce the waste in the water.
And last of all. Take everything, anyone says with a grain of salt. Theres a billion different ways to filter/raise fish etc. Just look at it like, that is what works for them. For every person who likes one way, theres a guy who likes the opposite. Just use this to get ideas and find out what works for you!
but will also take it all into vast consideration.